Language: C

#include <stdio.h>
#include <math.h>
int main(){
	int i,n;
	double x1,,x2,x3,y1,y2,y3,r;
	scanf("%d",&n);
	for (i=0; i<n; i++) {
		scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
		x1=(((x2*x2-x1*x1)+(y2*y2-y1*y1))*(y3-y2)-((x3*x3-x2*x2)+(y3*y3-y2*y2))*(y2-y1))/(2.0*((x2-x1)*(y3-y2)-(x3-x2)*(y2-y1)));		
		y1=(((y2*y2-y1*y1)+(x2*x2-x1*x1))*(x3-x2)-((y3*y3-y2*y2)+(x3*y3-x2*y2))*(x2-x1))/(2.0*((y2-y1)*(x3-x2)-(y3-y2)*(x2-x1)));
		r=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));
		if ((int)(x[0]*10000)%10>=5) x[0]+=0.001;
		if ((int)(y[0]*10000)%10>=5) y[0]+=0.001;
		if ((int)(r*10000)%10>=5) r+=0.001;
		printf("%.3f %.3f %.3f\n",x[0],y[0],r);
	}
	return 0;
}

Compiler output:
main.c: In function 'main':
main.c:5:19: error: expected identifier or '(' before ',' token
    5 |         double x1,,x2,x3,y1,y2,y3,r;
      |                   ^
main.c:8:53: error: 'x2' undeclared (first use in this function); did you mean 'x1'?
    8 |                 scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
      |                                                     ^~
      |                                                     x1
main.c:8:53: note: each undeclared identifier is reported only once for each function it appears in
main.c:8:57: error: 'y2' undeclared (first use in this function); did you mean 'yn'?
    8 |                 scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
      |                                                         ^~
      |                                                         yn
main.c:8:61: error: 'x3' undeclared (first use in this function); did you mean 'x1'?
    8 |                 scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
      |                                                             ^~
      |                                                             x1
main.c:8:65: error: 'y3' undeclared (first use in this function); did you mean 'yn'?
    8 |                 scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
      |                                                                 ^~
      |                                                                 yn
main.c:9:45: error: invalid operands to binary * (have 'double (*)(double)' and 'double (*)(double)')
    9 |                 x1=(((x2*x2-x1*x1)+(y2*y2-y1*y1))*(y3-y2)-((x3*x3-x2*x2)+(y3*y3-y2*y2))*(y2-y1))/(2.0*((x2-x1)*(y3-y2)-(x3-x2)*(y2-y1)));
      |                                          ~  ^
      |                                          |  |
      |                                          |  double (*)(double)
      |                                          double (*)(double)
main.c:10:31: error: invalid operands to binary * (have 'double (*)(double)' and 'double (*)(double)')
   10 |                 y1=(((y2*y2-y1*y1)+(x2*x2-x1*x1))*(x3-x2)-((y3*y3-y2*y2)+(x3*y3-x2*y2))*(x2-x1))/(2.0*((y2-y1)*(x3-x2)-(y3-y2)*(x2-x1)));
      |                            ~  ^
      |                            |  |
      |                            |  double (*)(double)
      |                            double (*)(double)
main.c:11:17: error: 'r' undeclared (first use in this function)
   11 |                 r=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));
      |                 ^
main.c:11:25: error: 'x' undeclared (first use in this function); did you mean 'x1'?
   11 |                 r=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));
      |                         ^
      |                         x1
main.c:11:49: error: 'y' undeclared (first use in this function)
   11 |                 r=sqrt((x[0]-x[1])*(x[0]-x[1])+(y[0]-y[1])*(y[0]-y[1]));
      |                                                 ^