In order to solve $ \color{blue}{ -75x^{4}+3x^{2} = 0 } $, first we need to factor our $ x^2 $.
$$ -75x^{4}+3x^{2} = x^2 \left( -75x^{2}+3 \right) $$$ x = 0 $ is a root of multiplicity $ 2 $.
The remaining roots can be found by solving equation $ -75x^{2}+3 = 0$.
$ -75x^{2}+3 = 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.