A converging lens and a diverging lens, separated by a distance of 20.5 cm, are used in combination. The converging lens has a focal length of 14.5 cm. The diverging lens is of unknown focal length. An object is placed 17.2 cm in front of the converging lens; the final image is virtual and is formed 11.4 cm before the diverging lens. What is the focal length of the diverging lens?