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.