Springs and Elastic Coollisions

[fibb]

Homework Statement

A ball of mass 2.5kg is moving to the left along a smooth, horizontal table at 3.5m/s. An idea spring (one that obeys Hooke's Law) with a spring contstant of k=1100N/m, and a relaxed length of 0.3m, is attached to its front. It collides head on with a 4.5 kg ball, initially at rest.
a) What is the velocity of each ball at minimum separation?
b) What is the change in total kinetic energy at minimum separation? as compared to initial K.E. of system?

Homework Equations

p=mv
Hooke's Law: Fx=kx
Ee=1/2kx^2
I know there's more equations needed

The Attempt at a Solution

For a, I found an expression of v1 and v2 with the momentum formula and conservation of momentum and tried to eliminate. But I got stuck at that part.
I don't really know how to do b.

 

[physixguru]

http://hyperphysics.phy-astr.gsu.edu/Hbase/elacol.html