Calculate Rotation Degree between Two Points

static double cosBetweenTwoVector(Point vec1,Point vec2){
    float divider = sqrt(pow(abs(vec1.x), 2)+ pow(abs(vec1.y),2)) * sqrt(pow(abs(vec2.x), 2) + pow(abs(vec2.y), 2));
    float cosin = (vec1.x*vec2.x)+(vec1.y*vec2.y)/ divider;
    return cosin;
}
          Point vec1 = Point(p1.x-p2.x,p1.y-p2.y);
          Point vec2 = Point(0,vec1.y);
          cout<< "Cosin= " << cosBetweenTwoVector(Point(0,3),Point(0,3));
          float rotationRadius = acos(cosBetweenTwoVector(vec1, vec2));
          float RAD2DEG = 180 / M_PI;
          float rotationDeg = rotationRadius * RAD2DEG;
          if (rotationDeg < 80 || rotationDeg > 100) {
             rotateImage(src, 90-rotationDeg, src);
          }
          cout<< "Rotatation: " << rotationDeg << endl;