If a function y=f(x) is differentiable at x=3 and f '(3)=5, find the limit of lim (x->3) (x^2-3x)/(f(3)-f(x))?

when people post math questions on yahoo answers, its the one question that they can't figure out no matter what. And instead of just leaving it and forgetting about it and hoping it doesn't show up on the exam, they post it on yahoo answers to try and find out the answer. What's so wrong about that?