A ball of mass $'m'$ moves towards a wall with a velocity $'u'$, the direction of motion making an angle $\theta$ with the surface of the wall and rebounds with the same period. The change in momentum of the ball during the collision is:

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 $\vec p_1$ and $\vec p_2$ is the momentum before and after the collision with wall.

Since , the angle with the wall is $\theta$, the change is only in $sin\theta$ component, which is across $\vec F_{wall}$.

Change in momentum$=m[usin\theta -(-usin\theta )]=2mu\ sin\theta$ away from the wall, as the final velocity is away from the wall.