I have been stuck on this for a while now and cant figure out what it is i am doing wrong HELP!!!

Question

A tennis ball with a velocity of 15.5 m/s is
thrown perpendicularly at a wall. After strik-
ing the wall, the ball rebounds in the opposite
direction with a speed of 9.3 m/s.
If the ball is in contact with the wall for
0.015 s, what is the average acceleration of
the ball while it is in contact with the wall?

Answer 1

aq = (V-Vo)/t = (-9.3-15.5) 0.015=-1660
m/s^2.

NOTE: The final velocity is in the opposite direction. Therefore, it is
NEGATIVE.

A 2nd interpretation:

While the ball is in contact with the wall, its' velocity is o.
a = (0-15.5) / 0.015 = 1033.3 m/s^2.