Hello,
For the first one :
You have $\displaystyle V=\frac 43 \pi r^3=321$
And you want $\displaystyle S=4 \pi r^2$
From the volume formula, you have :
$\displaystyle r^3=\frac{321 \cdot 3}{4 \pi}$
Hence $\displaystyle r=\sqrt[3]{\frac{321 \cdot 3}{4 \pi}}$
And substitute in the formula for S.
It's the exact same reasoning for the second one.