Physics Homework Help?

A railroad car of mass 12500 kg is released from rest in a railway switchyard and rolls to the bottom of a slope Hi =10.7 m below its original height. At the low point, it collides with and sticks to another car of mass 17500 kg. The two cars roll together up another slope and climb up to a height Hf above the low point, where they come to a stop before rolling back. Ignore the effects of friction and calculate Hf in m.

Hint:

If you solve the problem algebraicly first, it turns out to be a simple answer. However, you need to do it in steps.

First find v before the collision using

v = (2gh)1/2

from the conservation of the total mechanical energy.

Then use momentum conservation for a completely inelastic collision to find v'

Finally, you can use energy conservation again to calculate the final height

Hf = v'2/2g

I tried following them but it didn't work for me, I don't know if it's my math or what but I wanted to see someone else do it otherwise I wouldn't have asked the question.