Finding Vector after Reflection

So I have a vector, vector 1, hitting a wall. (This is partially a physics and computer science question, but contains a lot of trig.) This vector has VelocityX and VelocityY. The Normal angle of the wall can be found by a vector pointing to the opposite of the normal angle's direction (we can call this Vector Q). I need the vector hitting the wall to reflect across the normal angle (like a mirror and light).

My attempt was to take Arctan((VelocityY-Qy)/(VelocityX-Qx)) to find the angle of between Vector 1 and Vector Q, then take VelocityX*Cos(angle), and Sin for VelY.

But this didn't work. I tried many variations of this, but I just can't get it perfect.

All help appreciated! Thanks!