Spring problem. I thought I had it

[Wishingwell]

Homework Statement

To stretch a spring 3.30 cm from its unstretched length, 12.5 J of work must be done.
(a) What is the force constant of this spring?
(b) What magnitude force is needed to stretch the spring 3.30 cm from its unstretched length?
(c) How much work must be done to compress this spring 4.35 cm from its unstretched length?
(d) What force is needed to stretch it this distance?

Homework Equations

F = -kx

The Attempt at a Solution

Pre note: I feel as though if I can find part a I can find B-D without much trouble as it seems to be just plug and chug. Am I right to assume this?

I did the F =-kx (Where F = 12.5 and x = .033) and I got 378.7878... N/m which was wrong. Did I do anything wrong?

 

[rl.bhat]

12.5 J is the work done, not the force applied.
Use the expression for the energy stored in a spring to find k.