Average Acceleration During a Collision

[Ripcurl]

Homework Statement

A tennis ball with a velocity of 7.97 m/s to the right is thrown perpendicularly at a wall. After striking the wall, the ball rebounds in the opposite direction with a velocity of −5.07 m/s to the left. If the ball is in contact with the wall for 0.011 s, what is the average acceleration of the ball while it is in contact with the wall? Answer in units of m/s^2.

Homework Equations

The one that I assumed was the right equation was
ωf=ωi+αΔt
(ωf=final velocity) (ωi=initial velocity) (α=acceleration) (Δt =change in time)

The Attempt at a Solution

I just plugged the numbers into the formula and solved for α, but the answer was wrong. What I am I doing incorrectly?

 

[skeptic2]

What was your answer?