Answer
$P=\dfrac{k}{x^3}$
Work Step by Step
Recall:
If $y$ varies inversely as $x$, then, $y=\frac{k}{x}$ where $k$ is the constant of variation.
Since $P$ varies inversely as the cube of $x$, then, with $k$ as a constant of variation,
$$P=\frac{k}{x^3}$$