How Do Carts Behave and Springs Compress in an Elastic Collision?

[fa08ti]

In an elastic head-on collision, a 0.60 kg cart moving at 5.0 m/s (W) collides with a 0.80 kg cart moving at 2.0m/s (E). the collision is cushioned by a spring (k=1200 N/m)

a find the velocity of each cart after the collision

b find the maximum compression of the spring

ATTEMPT:

cart 1:
v1=( m1-m2/m1+m2)(v1)
v1= (0.6-0.8/0.6+0.8)(7)
v1= -1

then i did -1 + -1 = -3 m/s (E)

cart 2:

v2= (2m1/m1+m2) (v1)
v2= 6
6 + -2= 4 m/s (W)

i'm totally lost for part b

 

[dmriser]

It may help to recall the potential energy stored in a compressed spring is equal to

U = 0.5 * K * X^2

 

[cupid.callin]

you might want to check (a) again
i am getting different answers