A converging lens and a diverging lens, separated by a distance of 30.0 cm, are used in combination. The converging lens has a focal length of 15.0 cm. The diverging lens is of unknown focal length. An object is placed 60.0 cm to the left of the converging lens; the final image is formed 6.00 cm to the left of the diverging lens. Calculate the focal length of the diverging lens.