Hello, I need help with the lens experiment, the aim of which is to determine the focal length of a concave lens by combining it coaxially with a convex lens. Given the focal length of the CONVEX lens is 15cm. and the focal length of the CONCAVE lens is 30cm. Can you give me a set of reading, plotting which on the graph paper the slope of the st. line shall give the CONCAVE lens' focal length. Thanks in advance.