Replace h by 6:
Collect all terms on the LHS and factor out :
A product of 2 factors equals zero if at least one factor equals zero. By the 1st factor you'll get r = 0 which is a very, very slim cylinder, nearly invisible
With the 2nd factor you have to solve a quadratic equation in r:
You'll get 2 solutions for r. The negative value isn't very plausible here.