Language: C++

using namespace std;
 
int main(){
   
 int cnt;
    cin >> cnt;
   ??? double x1, y1, x2, y2, x3, y3;
    ???double a1, a2, b1, b2, c1, c2;
    ???double xp, yp, r;

    while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
        a1 = 2 * (x2 - x1);
        b1 = 2 * (y2 - y1);
        c1 = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
        a2 = 2 * (x3 - x1);
        b2 = 2 * (y3 - y1);
        c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
        xp = (double)((b1*c2 - b2*c1) / (a1*b2 - a2*b1) * 1000) / 1000.0;
        yp = (double)((c1*a2 - c2*a1) / (a1*b2 - a2*b1) * 1000) / 1000.0;
//

        r = hypot(xp - x1, yp - y1);
 
        cout<< setprecision(3)<< xp << " " << yp << " " << r << endl;
        cnt -= 1;
    }
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:6:5: error: 'cin' was not declared in this scope
    6 |     cin >> cnt;
      |     ^~~
a.cc:1:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
  +++ |+#include <iostream>
    1 | using namespace std;
a.cc:7:4: error: expected primary-expression before '?' token
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |    ^
a.cc:7:5: error: expected primary-expression before '?' token
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |     ^
a.cc:7:6: error: expected primary-expression before '?' token
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |      ^
a.cc:7:8: error: expected primary-expression before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |        ^~~~~~
a.cc:7:7: error: expected ':' before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |       ^~~~~~~
      |       :
a.cc:7:8: error: expected primary-expression before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |        ^~~~~~
a.cc:7:7: error: expected ':' before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |       ^~~~~~~
      |       :
a.cc:7:8: error: expected primary-expression before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |        ^~~~~~
a.cc:7:7: error: expected ':' before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |       ^~~~~~~
      |       :
a.cc:7:8: error: expected primary-expression before 'double'
    7 |    ??? double x1, y1, x2, y2, x3, y3;
      |        ^~~~~~
a.cc:8:5: error: expected primary-expression before '?' token
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |     ^
a.cc:8:6: error: expected primary-expression before '?' token
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |      ^
a.cc:8:7: error: expected primary-expression before '?' token
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |       ^
a.cc:8:8: error: expected primary-expression before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
a.cc:8:8: error: expected ':' before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
      |        :
a.cc:8:8: error: expected primary-expression before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
a.cc:8:8: error: expected ':' before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
      |        :
a.cc:8:8: error: expected primary-expression before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
a.cc:8:8: error: expected ':' before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
      |        :
a.cc:8:8: error: expected primary-expression before 'double'
    8 |     ???double a1, a2, b1, b2, c1, c2;
      |        ^~~~~~
a.cc:9:5: error: expected primary-expression before '?' token
    9 |     ???double xp, yp, r;
      |     ^
a.cc:9:6: error: expected primary-expression before '?' token
    9 |     ???double xp, yp, r;
      |      ^
a.cc:9:7: error: expected primary-expression before '?' token
    9 |     ???double xp, yp, r;
      |       ^
a.cc:9:8: error: expected primary-expression before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
a.cc:9:8: error: expected ':' before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
      |        :
a.cc:9:8: error: expected primary-expression before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
a.cc:9:8: error: expected ':' before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
      |        :
a.cc:9:8: error: expected primary-expression before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
a.cc:9:8: error: expected ':' before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
      |        :
a.cc:9:8: error: expected primary-expression before 'double'
    9 |     ???double xp, yp, r;
      |        ^~~~~~
a.cc:11:30: error: 'x1' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                              ^~
a.cc:11:36: error: 'y1' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                                    ^~
a.cc:11:42: error: 'x2' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                                          ^~
a.cc:11:48: error: 'y2' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                                                ^~
a.cc:11:54: error: 'x3' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                                                      ^~
a.cc:11:60: error: 'y3' was not declared in this scope
   11 |     while (cnt > 0 && cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3) {
      |                                                            ^~
a.cc:12:9: error: 'a1' was not declared in this scope
   12 |         a1 = 2 * (x2 - x1);
      |         ^~
a.cc:13:9: error: 'b1' was not declared in this scope
   13 |         b1 = 2 * (y2 - y1);
      |         ^~
a.cc:14:9: error: 'c1' was not declared in this scope
   14 |         c1 = x1 * x1 - x2 * x2 + y1 * y1 - y2 * y2;
      |         ^~
a.cc:15:9: error: 'a2' was not declared in this scope
   15 |         a2 = 2 * (x3 - x1);
      |         ^~
a.cc:16:9: error: 'b2' was not declared in this scope
   16 |         b2 = 2 * (y3 - y1);
      |         ^~
a.cc:17:9: error: 'c2' was not declared in this scope
   17 |         c2 = x1 * x1 - x3 * x3 + y1 * y1 - y3 * y3;
      |         ^~
a.cc:18:9: error: 'xp' was not declared in this scope
   18 |         xp = (double)((b1*c2 - b2*c1) / (a1*b2 - a2*b1) * 1000) / 1000.0;
      |         ^~
a.cc:19:9: error: 'yp' was not declared in this scope
   19 |         yp = (double)((c1*a2 - c2*a1) / (a1*b2 - a2*b1) * 1000) / 1000.0;
      |         ^~
a.cc:22:9: error: 'r' was not declared in this scope
   22 |         r = hypot(xp - x1, yp - y1);
      |         ^
a.cc:22:13: error: 'hypot' was not declared in this scope
   22 |         r = hypot(xp - x1, yp - y1);
      |             ^~~~~
a.cc:24:9: error: 'cout' was not declared in this scope
   24 |         cout<< setprecision(3)<< xp << " " << yp << " " << r << endl;
      |         ^~~~
a.cc:24:9: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:24:16: error: 'setprecision' was not declared in this scope
   24 |         cout<< setprecision(3)<< xp << " " << yp << " " << r << endl;
      |                ^~~~~~~~~~~~
a.cc:1:1: note: 'std::setprecision' is defined in header '<iomanip>'; this is probably fixable by adding '#include <iomanip>'
  +++ |+#include <iomanip>
    1 | using namespace std;
a.cc:24:65: error: 'endl' was not declared in this scope
   24 |         cout<< setprecision(3)<< xp << " " << yp << " " << r << endl;
      |                                                                 ^~~~
a.cc:1:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
  +++ |+#include <ostream>
    1 | using namespace std;