lens (optics problem)

[supermenscher]

I am having some trouble with this one...any help would be appreciated.

A 31.8cm focal length converging lens is 21.6cm behind a diverging lens. Parallell light strikes the diverging lens. After passing through the converging lens, the light is again parallel. What is the focal length of the diverging lens.

I used the lens equation to find the distance of the image and got 12.86cm, then plugged that in to the lens equation for a diverging lens and got -8.061 cm. However, that answer is incorrect. Any help would be appreciated.

[Doc Al]

There are several ways to solve this. Let's start this way: assume the focal length of the diverging lens is -x. If so, where will image from the diverging lens be formed? (Hint: where does parallel light get focused?)

Then realize that the image formed by the first lens acts as the object for the second lens.