#include <iostream>
#include <string>
using namespace std;
int T,a,b;
string s;
int main()
{
cin>>T;
for(int t=0; t<T; t++)
{
cin>>a>>b>>s;
for(auto iter=s.begin(); iter != s.end(); ++iter)
{
cout<<(char) ((a*((*iter)-'A')+b)%26 + 'A');
}
cout<<endl;
}
}