How to rearrange the equation?

I've been trying (unsuccessfully!) to rearrange the following equation so that H is the subject:

mgH = [k(H - L)^2] / 2

(Where ^2 means squared.) I also have some test data which shows that if m = 50, g = 9.8, k = 40 and L = 20, H should equal 57.5, or thereabouts