How to Find Final Velocities in an Elastic Collision?

[ineedhelp2395]

Mass of cart A: 1kg
Mass of Cart B: 3Kg
Spring potential energy: 24 Joules
Two carts are pushed apart when the spring is released.
Thats all the given and I'm suppose to find the final velocity.

I have no idea how to get started.
For me to use Es=(1/2)K*DeltaX^2, i don't have delta x or spring konstant.
For me to use P=M*V, Mass is not give and i don't know the momentum.
The only other equation i have i think that is related to this topic is the impulse equation. F*deltaT= Impulse

 

[ineedhelp2395]

When i drew the momentum bar graph, i set spring as the impulse so there was no momentum in beging but some at the end.

 

[ineedhelp2395]

Now i figured out using lol diagram that es would be equal to ek so then Ek=.5*m*V^2 and found the velocity not using the momentum. Anyone think what i did was wrong?