I am having trouble with my homework. I know the answer but I can't figure out how to get there. The question is "An image of a flower is produced on a screen by a lens 1.60m from the screen. The image of the flower has a linear magnification of -2.5. What is the focal point of the lens?" I know the 1/o + 1/i = 1/f. I also know that m = -i/o. Also, the image distance 1.6m. So, shouldn't -2.5=-1.6/o? But when I do this I get the wrong answer! Please help me!