Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

A simple camera telephoto lens consists of two lenses. The objective lens has a focal length f1 = +40.7 cm. Precisely 34.5 cm behind this lens is a concave lens with a focal length f2 = -13.1 cm. The object to be photographed is 3.75 m in front of the objective lens. How far behind the concave lens should the film be placed?

This is asking for the distance of the last image that is formed by the concave lense right? So I find the image made my the objective lens using (1/f) = 1/d(object) + 1/d(image) then I plug in and solve for d(image). Then use that as d(object) for the concave lens.
I got 1/-13.1 = -(1/d(image for from first step - distance between lens) + (1/d(image))
But somehow it's wrong. and I'm not understand what I'm doing wrong... i have two problems like this and i got them wrong on both problems. Can someone help?

Thank you in advance