Answer
See below.
Work Step by Step
$x^4\lt 9x^2\\x^4-9x^2\lt0\\x^2(x^2-9)\lt0$
We know that $(x-k)^2\geq0$ for all $k$.
So here for the inequality to be true, $x^2-9\lt0\\x^2\lt9$
$x^2\lt9$ when $-3\lt x\lt3$
So the solution is $-3\lt x\lt3$
