Let the angle that velocity vector makes with the wall surface be 30 deg.
The velocity vector makes 60 deg. with the perpendicular to the wall at the point of contact.
The horizontal and vertical components before the collision are:
V Cos60° , V Sin 60°
After the rebound, the components are :
- V Cos 60° and V Sin 60°
Only the horizontal component changes. So change in momentum is M *2 V Cos 60° , so that is M * V.