A 1.50 3 103 - kg car starts from rest and accelerates uniformly to 18.0 m/s in 12.0 s. Assume that air resistance remains constant at 400 N during this time. Find the average power developed by the engine

In this question, why do we not find the distance by using the speed formula? If we know that v = displacement over time, why do we have to use displace = Vit+1/2at^2 ?