power of a car freewheeling?

a car of mass 3000kg wil just run down a slope of inclination sin-1(1/20) under its own weight.assuming constant resisting forces and a constant tractive force due to the engine determine the power developed when a speed of 15 m/s is attained from rest in a time of 4 mins,when moving on a level road.

I can't work this out.can anyone explain it?
Do I need the slope part at all?

This is a work,energy and power question.

The problem doesn't make sense to me. You say the car goes downhill "under its own weight" and use, in the title, "freewheeling" but also say "constant tractive force due to the engine". If the engine is applying any tractive force at all the car is not moving "under its own weight" nor is it "freewheeling".