Question 775591
I assume that your equation looks like:
S(2r) = {{{(1/3)(pi)r^2(2r)}}}
 
(1) r^2(2r) = 2r^3
{{{(1/3)(pi)(2r^3) = 0)}}}
 
(2) Divide both sides by 2(pi)/3
{{{r^3 = 0)}}}
 
(3) Cube root both sides.  You are left with:
r = 0