How to build a car propelled from suspended mass?

For my physics class I need to build a car that will travel 6 m and lightly touch a wall. There will be an egg on the front of the car and it can not break. The car must be self propelled from a suspended mass and pulley system. Any design ideas, pictures, or tips?
How to build a car propelled from suspended mass?
I assume your school provides %26quot;Erector Set%26quot; components to build a car with wheels, axles, chassis, and a tower. You need a tower roughly 1.5 meter tall on the car. Suspend a pulley system with a mechanical advantage of 6:1. Wrap 6 meters of string around the rear axle then up through the pully system. Hang masses off the pully system until the car rolls smoothly.



THEN: Test it and trim the string until it just goes 6 meters, starts to lift the masses again and stops.