Answer:
3 m/s^2
Explanation:
You are given their initial velocity and their final velocity, as well as the time.
This allows you to use the equation for acceleration, a = Δv/Δt (change in velocity over change in time):
vfinal - vinitial = Δv
(18 m/s - 9 m/s)/ 3 sec = 3 m/s^2
