In order to solve $ \color{blue}{ x^{4}-x^{3}+3x = 0 } $, first we need to factor our $ x $.
$$ x^{4}-x^{3}+3x = x \left( x^{3}-x^{2}+3 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ x^{3}-x^{2}+3 = 0$.
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using qubic formulas.