#### Answer

The constant of variation is $\frac{1}{3}$.
The variation equation is $y=\frac{1}{3}x^3$.

#### Work Step by Step

$y=kx^3$
$y=9; x=3$
$9=k(3)^3$
$9=k(27)$
$k=\frac{1}{3}$
$y=\frac{1}{3}x^3$
The constant of variation is $\frac{1}{3}$.
The variation equation is $y=\frac{1}{3}x^3$.