In order to solve $ \color{blue}{ -4x^{9}+5x^{4} = 0 } $, first we need to factor our $ x^4 $.
$$ -4x^{9}+5x^{4} = x^4 \left( -4x^{5}+5 \right) $$$ x = 0 $ is a root of multiplicity $ 4 $.
The remaining roots can be found by solving equation $ -4x^{5}+5 = 0$.
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using Newton method.