In order to solve $ \color{blue}{ x^{4}-9x^{2} = 0 } $, first we need to factor our $ x^2 $.
$$ x^{4}-9x^{2} = x^2 \left( x^{2}-9 \right) $$$ x = 0 $ is a root of multiplicity $ 2 $.
The remaining roots can be found by solving equation $ x^{2}-9 = 0$.
$ x^{2}-9 = 0 $ is a quadratic equation.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this equation.