Determining Spring Constant After Collision

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Homework Statement

1.2 kg cart slides down frictionless ramp from a height of 1.8m and then onto a horizontal surface where it has a head-on elastic collision with a stationary 2.0kg cart cushioned by an ideal Hooke's Law spring. Maximum compression of spring is 2.0 cm
Determine the spring Constant

Homework Equations

The Attempt at a Solution

Em1=Em2
mgh=1/2kx^2
But this did not get me the answer

 

[PeterO]

Homework Statement

1.2 kg cart slides down frictionless ramp from a height of 1.8m and then onto a horizontal surface where it has a head-on elastic collision with a stationary 2.0kg cart cushioned by an ideal Hooke's Law spring. Maximum compression of spring is 2.0 cm
Determine the spring Constant

Homework Equations

The Attempt at a Solution

Em1=Em2
mgh=1/2kx^2
But this did not get me the answer

Don't forget that when the spring is at maximum compression, the (previously) stationary cart has already begun to move, so only part of the Initial energy of other Cart is stored in the spring.