Language: C++

#include <iostream>
#include <string>
#define rep(i) for(I64 i=0;;++i)
#define repn(i,n) for(I64 i=0;i<(n);++(i))
using namespace std;

bool coprime(int a, int b){
	int r, temp;
	if(a < b){
		temp = a;
		a = b;
		b = temp;
	}
	do{
		r = a % b;
		a = b;
		b = r;
	}while(r);
	if(a == 1) return true;
	else return false;
}

int main(){
	char t[26];

	int n;
	cin >> n;

	scanf("%*c");

	repn(i, n){
		string input;
		getline(cin, input);
		int len = input.size();

		bool flag = false;
		rep(a){
			if(coprime(26, a+1)){
				repn(b, 26){
					string res = "";
					repn(r, 26){
						t[r] = ((a+1) * r + b) % 26 + 'a';
					}
					repn(j, len){
						if(input[j] == ' ') res += ' ';
						else{
							repn(k, 26){
								if(input[j] == t[k]){
									res += k + 'a';
									break;
								}
							}
						}
					}
					if(res.find("this") != string::npos || res.find("that") != string::npos){
						cout << res << endl;
						flag = true;
						break;
					}
				}
				if(flag) break;
			}
			if(flag) break;
		}
	}
}

Compiler output:
a.cc: In function 'int main()':
a.cc:4:23: error: 'I64' was not declared in this scope
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                       ^~~
a.cc:31:9: note: in expansion of macro 'repn'
   31 |         repn(i, n){
      |         ^~~~
a.cc:31:14: error: 'i' was not declared in this scope
   31 |         repn(i, n){
      |              ^
a.cc:4:31: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                               ^
a.cc:37:21: error: expected ';' before 'a'
   37 |                 rep(a){
      |                     ^
a.cc:3:24: note: in definition of macro 'rep'
    3 | #define rep(i) for(I64 i=0;;++i)
      |                        ^
a.cc:37:21: error: 'a' was not declared in this scope
   37 |                 rep(a){
      |                     ^
a.cc:3:31: note: in definition of macro 'rep'
    3 | #define rep(i) for(I64 i=0;;++i)
      |                               ^
a.cc:39:38: error: expected ';' before 'b'
   39 |                                 repn(b, 26){
      |                                      ^
a.cc:4:27: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                           ^
a.cc:39:38: error: 'b' was not declared in this scope
   39 |                                 repn(b, 26){
      |                                      ^
a.cc:4:31: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                               ^
a.cc:41:46: error: expected ';' before 'r'
   41 |                                         repn(r, 26){
      |                                              ^
a.cc:4:27: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                           ^
a.cc:41:46: error: 'r' was not declared in this scope
   41 |                                         repn(r, 26){
      |                                              ^
a.cc:4:31: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                               ^
a.cc:44:46: error: expected ';' before 'j'
   44 |                                         repn(j, len){
      |                                              ^
a.cc:4:27: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                           ^
a.cc:44:46: error: 'j' was not declared in this scope
   44 |                                         repn(j, len){
      |                                              ^
a.cc:4:31: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                               ^
a.cc:47:62: error: expected ';' before 'k'
   47 |                                                         repn(k, 26){
      |                                                              ^
a.cc:4:27: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                           ^
a.cc:47:62: error: 'k' was not declared in this scope
   47 |                                                         repn(k, 26){
      |                                                              ^
a.cc:4:31: note: in definition of macro 'repn'
    4 | #define repn(i,n) for(I64 i=0;i<(n);++(i))
      |                               ^