Optics Q; Bessel's method

Perses

I'm having a bit of trouble finding a place to start in one of my optics questions. I have to find Bessel's method for focal length of a lens. Any pointers on where to begin or a general direction, that would be greatly appreciated! Here's the question:

A thin lens is moved along the optical axis between a fixed object and a fixed image screen. The object and image positions are separated by a distance L that is more than four times the focal length of the lens. Two positions of the lens are found for which an image is in focus on the screen, magnified in one case and reduced in the other. If the two lens positions differ by distance D, show that the focal length of the lens is given by;

f = (L^2 - D^2)/4L.

Perses

any help anyone could offer would be appreciated, thnx