1. The problem statement, all variables and given/known data I need to find q in terms of f and p. 2. Relevant equations I am dealing with this equation at the moment: 1/p + 1/q = 1/f. 3. The attempt at a solution I think I should isolate the q, so that I have the variables p and f on one side of the equals sign, and q on the other side of the equals sign. Am I correct in thinking this?