cylinder question

The cylinder has a volume of 30m^3
What dimensions will use the least material...

SO we have
V = pi.r^2.h
SA = 2pi.r^2.h + 2pi.r.h

i try to find the surface area in terms of one variable

so i get h = 30 / pi.r^2 and sub in

SA = 2pi.r^2 + 2pi.r.(30/pi.r^2)
i then get...
SA = 2pi.r^2 + 60r^-1

find the first derivative

dSA/dr = 4pi.r - 60r^-2
to find the turning points and thus the minimum find r when dSA/dr = 0

0 = 4pi.r - 60r^-2 and thats where i get stuck. im not even sure if im on the right track, so feel free to shut me down.

sorry for a long question but any help helps.
thanks