Solving for the Speed of a Box on an Incline

[bodensee9]

Hello:

Can someone help with the following:
A 2.0 kg box on a frictionless incline of angle theta = 40 is connected by a cord that runs over a pulley to a light spring of spring constant k = 120N/m. The box is released from rest when the spring is unstretched. Assume pulley is massless and frictionless. I've attached drawing for clarification.

Find speed of box when it has moved 10 cm.

So can I assume that since the box has moved 10 cm, then the spring also has to move 10 cm? Then I can do
1/2k(.1)^2 = 1/2mv^2 - mg(10sin40).
Thanks.

 

[G01]

I can't the pic yet, but from the description, I think you're on the right track trying to do this with energy.