A 45.0 g Super Ball traveling at 30.0 m/s bounces off a brick wall and rebounds at 17.0 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 4.50 ms, what is the magnitude of the average acceleration of the ball during this time interval? (Note: 1 ms = 10-3 s.) please explain...i dont get it!!