The surface area S of sphere is given by
[tex]S=4\pi r^2[/tex]In our case
[tex]S=324\pi[/tex]Then, we have
[tex]324\pi=4\pi r^2[/tex]By moving 4Pi to the left hand side, we have
[tex]\frac{324\pi}{4\pi}=r^2[/tex]We can cancel Pi out in numerator and denominator, then ,we get
[tex]\begin{gathered} \frac{324}{4}=r^2 \\ 81=r^2 \\ \text{therefore} \\ r=\sqrt[]{81} \end{gathered}[/tex]and the answer is r= 9 kilometers, that is, the radius is 9 km
