In order to solve $ \color{blue}{ 5x^{3}+4x^{2}-x = 0 } $, first we need to factor our $ x $.
$$ 5x^{3}+4x^{2}-x = x \left( 5x^{2}+4x-1 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ 5x^{2}+4x-1 = 0$.
$ 5x^{2}+4x-1 = 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.