Answer
$(a)$ $P=ks^3$
$(b)$ $k=0.012$
$(c)$ $P= 324watts$
Work Step by Step
$(a)$ According to the problem, we have $C=ks^3$
Where $k$ is the constant of proportionality.
$(b)$
$P=96 watts$
$s=20mi/h$
$k$ is unknown
$96=k\times20^3$
$8000k=96$
$k=0.012$
$(c)$
$s=30mi/h$
$P$ is unknown
$P=0.012\times27000 = 324watts$