In order to solve $ \color{blue}{ -6x^{6}+16x^{5}-9x^{4} = 0 } $, first we need to factor our $ x^4 $.
$$ -6x^{6}+16x^{5}-9x^{4} = x^4 \left( -6x^{2}+16x-9 \right) $$$ x = 0 $ is a root of multiplicity $ 4 $.
The remaining roots can be found by solving equation $ -6x^{2}+16x-9 = 0$.
$ -6x^{2}+16x-9 = 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.