What is the focal length of the diverging lens?

A 30.6 cm focal-length converging lens is 20.0 cm behind a diverging lens. Parallel 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? (Hint: First draw a ray diagram.)

Diagram:

http://a323.yahoofs.com/mbl/pics426632/426632.jpg?mlAI.sLBvh19auKM

Let f be the unsigned focal length of the diverging lens.
The diagram shows parallel light entering the diverging lens from the left.
Between the two lenses, the light appears to diverge from a point X distance f to the left of the first lens.
As the final image is at infinity:
f + 20cm = 30.6cm
f = 10.6cm.