[SOLVED] Lens Problem, Help from Anyone! Thank you, I appreciate it very much! 1. The problem statement, all variables and given/known data An object is placed 30 mm in front of a lens. An image of the object is located 90 mm behind the lens. a) What is the focal lenght of the lens? b) On an axis (the x-axis) draw the lens at position x=0. draw at least two rays and locate the image to show the situation described above. 2. Relevant equations 1/f = 1/Do + 1/Di 3. The attempt at a solution if i use -90mm then f= -45.