Hi, and welcome to the forum.
You are told that the can has to have a prescribed volume V. Using this fact, you can express, say, h as a function of an unknown r and a known constant V.I don't know how r and h relate to each other in this problem.
After that, find the total area of metal that is needed to make a can of radius r and height h (note that the top and bottom are squares, not circles). Substitute h from the previous paragraph to get a function of r only. Using differentiation, find r in terms of V that gives the minimum of the function.
Now you should have an equality on r and V. Substituting , you get an equation on r and h, from where you can get the required ratio r/h.