Answer
$$x∈[−3,3]$$
Also see the image below.
Work Step by Step
$4x^2(x^2-9)\leq0$
Graphing the equation $4x^2(x^2-9)$ gives us the Blue graph shown on the image above.
We need to locate the region where the graph has value less than or equal to $0$, that is the region below and including, $y=0$ ($x$-axis) line. So we have the solution:
$x∈[−3,3]$
