Here's the problem:

Now, I know the volume of the space enclosed by a cylinder is $\displaystyle V= \pi r^2h$What are the dimensions of the soup can of greatest volume that can be made with 50 square inches of tin? (The entire can, including the top and bottom, are made of tin.)

and what's its volume?

What I don't know is how to differentiate this in respect to it's height. Up to now, I've just had to differentiate a few x's. Now, I guess I would just imagine the h as an x, but how would I differentiate for r? I tried isolating the r, but then I end up with a V on the other side. Help me, brothers.