Question on Verbal p.14 #5 (6th edition)

The question is as follows:
Elastic potential energy in a spring is directly proportional to the square of the displacement of one end of the spring from its rest position while the other end remains fixed. If the elastic potential energy in the spring is 100J when it is compressed to half its rest length, what is the energy when it is compressed to one fourth its rest length.
I am trying to figure what the equation is so I can understand how they are deriving at the answer.
If anyone has done this problem yet and understands it, holler.

I think the equation you are thinking of is PE=1/2kx^2
The force is equal to F=-kx.
Is this question in the verbal section??