A 3.4 kg-object moving at 9.9 m/s perpendicular to a wall rebounds from the wall at 4.5 m/s. The magnitude of the change in the momentum of the ball is:

p1 = M*V = 3.4 * 9.9 = 33.7. = Momentum.

p2 = 3.4 * (-4.5) = -15.3.

p2-p1 = -15.3 - 33.7 = -49.
Magnitude = 49.

The negative sign means the momentum
decreased.