In order to solve $ \color{blue}{ x^{8}-x^{6}-2x^{4} = 0 } $, first we need to factor our $ x^4 $.
$$ x^{8}-x^{6}-2x^{4} = x^4 \left( x^{4}-x^{2}-2 \right) $$$ x = 0 $ is a root of multiplicity $ 4 $.
The remaining roots can be found by solving equation $ x^{4}-x^{2}-2 = 0$.
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas