In order to solve $ \color{blue}{ -4x^{4}+36x^{3}+108x = 0 } $, first we need to factor our $ x $.
$$ -4x^{4}+36x^{3}+108x = x \left( -4x^{3}+36x^{2}+108 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ -4x^{3}+36x^{2}+108 = 0$.
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using qubic formulas.