$P=4\pi R^2$  

$V=\frac{4}{3}\pi R^3$  

$a)$  

$\frac{4}{3}\pi R^3=36\pi \mid :\pi$  

$\frac{4}{3}{R}^3=36\mid \cdot \frac{3}{4}$  

$R^3={\sout{36}}^9\cdot \frac{3}{{\sout{4}}^1}$  

$R^3=27$  

$R=\sqrt[3]{27}=3cm$