Elastic Potential Energy Problem

[phizics09]

Homework Statement

A 1.2 kg spring laboratory cart is held against a wall. The spring constant is 65.0 N/m. The spring is compressed 8.0 cm when held against the wall. What is the compression of the spring when the cart's velocity is 42.0 cm/s?

Homework Equations

(1/2)mv^2=(1/2)kx^2

The Attempt at a Solution

(1.2)(0.42)^2=65(x)^2
x=5.71 cm

But the answer says 5.6 cm, so I think I did something wrong.

 

[ss85]

Since the cart is starting out already compressed to 8cm, you would have to consider that initial displacement in your equation. Subtract the final elastic potential energy from the original (x=8cm) elastic potential energy on the right side. Do that and you should get 5.6cm.