A 15-cm -focal-length converging lens is 18 cm to the right of a 7.0-cm -focal-length converging lens. a 2.3-cm -tall object is distance l to the left of the 7.0-cm -focal-length lens. For what value of L is the final image of this two-lens system halfway between the two lenses?
Express your answer to two significant figures and include the appropriate units.