You measure the focal length of a concave lens by first forming a real image of a light source using a convex lens. The image is formed on a screen 18 cm from the lens. You then place the concave lens halfway between the convex lens and the screen. To obtain a sharp image, you need to move the screen 15 cm further away from the lenses. What is the focal length of the concave lens? Express your answer to two significant figures and include the appropriate units.