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Homework Statement:

A 0.29kg cart and a 0.19kg cart are held together with a compressed spring between them. When they are released, the 0.29kg cart moves at 1.1 m/s to the right.
How much elastic potential energy was stored in the spring before the release?
Relevant Equations:

U = (1/2)kx^2
k = spring force constant
x = stretch length
Is this the correct formula to use, if not what is. I really just don't know where to start. I really just need to know where and how to start.