#### Answer

(a) $P=ks^{3}$
(b) $P=0.012s^{3}$
(c) $324$ Watts

#### Work Step by Step

(a) Since $P$ is directly proportional to the cube of $s$, we have:
$P=ks^{3}$
(b) We are given: $P=96$ and $s=20$, so we have:
$96=k* 20^{3}$
$k=0.012$
Therefore:
$P=0.012s^{3}$
(c) We are given $s=30$ and need to find $P$:
$P=0.012*30^{3}=324$ Watts.