Calculating Average Acceleration of a Bouncing Super Ball

[MG5]

A 45.0-g Super Ball traveling at 30.0 m/s bounces off a brick wall and rebounds at 20.5 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.70 ms, what is the magnitude of the average acceleration of the ball during this time interval?

Apparently the answer is 13600 m/s squared.

No idea how to get that though.

 

[voko]

What is average acceleration?

 

[Ripe]

A 45.0-g Super Ball traveling at 30.0 m/s bounces off a brick wall and rebounds at 20.5 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.70 ms, what is the magnitude of the average acceleration of the ball during this time interval?

Apparently the answer is 13600 m/s squared.

No idea how to get that though.

I'm a bit sleepy at the moment and might have overlooked something, but it doesn't make sense that the acceleration is positive when the ball slows down by 9,5m/s..