Suppose you increase your walking speed from 7 m/s to 15 m/s in a period of 2 s. what is your acceleration

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