Answer:
1200 m/s2
Explanation:
a = (v-u) / t
where a = acceleration
      v = final velocity
      u = initial velocity
      t = time taken
As velocity is a vector it is having a direction. So as the ball bumps out in the opposite direction which it was thrown, the sign of the velocities must be different from each other.
Consider u = -13.7 m/s
then     v = +11.5 m/s
        t =0.021 s
Applying in the above equation,
a = Â (11.5-(-13.7))/0.021 = 25.2/0.021 = 1200 m/s2