It's always a good idea to start with the definition of the thing you're trying to find.
This problem is just trying to find out whether you KNOW the definition of acceleration. You may know it, but you haven't used it yet.
Average acceleration = (change in velocity) divided by (total time).
Change in velocity = (end value) minus (start value)
Change in velocity = (20m/s) - 0
Change in velocity = 20 m/s
Time = 10 s
Average acceleration = (20m/s)/(10s)
Average acceleration = 2 m/s^2
