Edexcel Mechanics 2 question

A cyclist starts from rest and freewheels down a hill inclined at arcsin 1/20 to the horizontal. After travelling 80 meters the road becomes horizontal and the cyclist travels a further 80 meters before coming to rest without using the brakes. The cyclist and the bike have a combined mass of 80kg. Find the resistive force (assumed constant throughout the motion).

Anyone fancy a crack at that badboy? I don't see where I'm going wrong.