Centripetal acceleration on a frictionless circular track involving springs. Solution in JPG format.

A car is to be launched down a frictionless track by compressing a spring behind the car and releasing it - much the same as a pinball machine. If the track contains a circualr vertical loop of radius R, how far must the spring be compressed (x) in order for the car to stay on the track?
Given: mass of the car m=840kg. R=6.2m