Hey folks, I've got a problem here that I can't figure out how to start. Just a pointer in the right direction would be great.

Havn't yet hit a problem where I need to work backwards to find this. I'm not sure how to start, because even if I do find the spring constant and the force, I can only find the displacement from the natural length, not the natural length itself. What am I missing?What is the natural length of a spring (in cm) which needs 6 joules of work to stretch it to a length of 10 cm to 12 cm, and another 10 joules to stretch it from 12 cm to 14 cm

Thanks!