In order to solve $ \color{blue}{ -3x^{4}-18x^{3}+15x^{2} = 0 } $, first we need to factor our $ x^2 $.
$$ -3x^{4}-18x^{3}+15x^{2} = x^2 \left( -3x^{2}-18x+15 \right) $$$ x = 0 $ is a root of multiplicity $ 2 $.
The remaining roots can be found by solving equation $ -3x^{2}-18x+15 = 0$.
$ -3x^{2}-18x+15 = 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.