What is the average acceleration of a jet ski that goes from rest to 10 m/s west in 4 s? take the direction west to be positive, and east to be negative.

From the question we come to know several important information's and based on them we can reach the desired result.
Initial speed = 0 m/s
Final speed = 10 m/s
Initial time = 0 sec
Final time = 4 sec
Then
Average acceleration = (Final speed - Initial speed)/(Final time - Initial time)
                                   = (10 - 0)/(4 - 0)
                                   = 10/4
                                   = 2.5 m/s^2
So the average acceleration is 2.5 m/s^2. I hope this has made it clear to you about the procedure of solving such problems.