how to find the average acceleration?

Question

lets just say that a car travels from rest in a distance of 400m in 16 seconds?

Answer 1

Assume a constant acceleration - what you find, then, will be the average. You know the formula

s = v0 t + (1/2) a t^2

You know s = 400 m, v0 = 0 and t = 16 s. Solve for a, and Bob's your Uncle.