In order to solve $ \color{blue}{ 3x^{5}+28x^{4}-8x^{3}-9x^{2} = 0 } $, first we need to factor our $ x^2 $.
$$ 3x^{5}+28x^{4}-8x^{3}-9x^{2} = x^2 \left( 3x^{3}+28x^{2}-8x-9 \right) $$$ x = 0 $ is a root of multiplicity $ 2 $.
The remaining roots can be found by solving equation $ 3x^{3}+28x^{2}-8x-9 = 0$.
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using qubic formulas.