Answer
The average acceleration is 3.67 $m/s^2$ and the time it takes is 3.13 seconds.
Work Step by Step
$a = \frac{v^2-v_0^2}{2x} = \frac{11.5^2}{2\times18.0} = 3.67 ~m/s^2$
$t = \frac{v-v_0}{a} = \frac{11.5}{3.67} = 3.13 ~s$
The average acceleration is 3.67 $m/s^2$ and the time it takes is 3.13 seconds.
