Answer :
With "toward the wall" designated as the positive direction, the average acceleration felt the ball in the 0.0158 s of contact with the wall is
[tex]a_{\rm ave} = \dfrac{-26.412\frac{\rm m}{\rm s} - 28.4\frac{\rm m}{\rm s}}{0.0158\,\mathrm s} ≈ \boxed{-3470 \dfrac{\rm m}{\mathrm s^2}}[/tex]