Knowing the magnitude of the initial velocity and the magnitude of the final velocity of a body, we can calculate its average acceleration if we know how much time elapsed in the change of velocity.
So:
a = Δv / Δt
a = (v2-v1) / t2-t1
a = (15-7) / 2
a = 8/2
a = 4m / s²
Finally the average acceleration with which it moves is 4m / s²
