As the ball hits the wall at an angle and rebounds, there is a change in the momentum only along the direction perpendicular to the wall. The change along the wall is zero. In diagram →p1 and →p2 is the momentum before and after the collision with wall.
Since , the angle with the wall is θ, the change is only in sinθ component, which is across →Fwall.
Change in momentum=m[usinθ−(−usinθ)]=2mu sinθ away from the wall, as the final velocity is away from the wall.