You went to all that trouble to create f(r) = Cost of Producing the can as a function of r, and now you don't know what to do with it? Come on! Make the leap!!

What section are you in? I'm guessing "applications of the derivative"?

Find f'(r) and see if it is ever zero (0).

1) Make sure it is a minimum and not a maximum

2) Don't forget to check endpoints, since the derivative doesn't know about those. You have 0 < r < 500/pi at least. It is likely you can narrow the Domain much more than that with a little thought.