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See diagram enclosed.

Let us say the ball is moving in the direction of positive X axis.
Momentum of ball before colliding with the wall = mass * velocity = p1 = m u
 Momentum is in the direction of +ve x axis.

After rebounding let us say the ball moves back with a speed v. Since the ball moves in the negative x direction (after reflection) , we denote it as -v.
Its momentum after rebounding is p2 = =  - mv       

Change in the momentum during the rebounding is
              = final momentum -  initial momentum  = p2 - p1 =
               =  (- m v ) - (m u)  = -  m v - m u = -m ( v+u)
The change in momentum is a vector quantity and is in - ve x direction.