In case of their in-built c++ function where vector has to be returned, this code by me worked:

vector findPoint(int px, int py, int qx, int qy) {

 int rx=2*qx-px;
 int ry=2*qy-py;


 vector<int> v;
 v.push_back(rx);
 v.push_back(ry);
 return v;

}