V = πh^2(R - h/3)
V/πh^2 = R - h/3
3[V/πh^2] = 3R - h
([3V/πh^2] + h) = 3R
([3V/πh^2] + h)/3 = R
You can simplify the left side a bid more but, for the most part, we have isolated R (solved for R).
