we have
S= 4*(pi)*r^2
solve for r
That means ------> isolate the variable r
step 1
Divide by 4*(pi) both sides
so
S/(4*pi)=r^2
step 2
Square root both sides
so
[tex]\begin{gathered} r=\sqrt[]{\frac{S}{4\pi}} \\ \text{simplify} \\ r=\frac{1}{2}\sqrt[]{\frac{S}{\pi}} \end{gathered}[/tex]