In order to solve $ \color{blue}{ 12x^{4}-20x^{3} = 0 } $, first we need to factor our $ x^3 $.
$$ 12x^{4}-20x^{3} = x^3 \left( 12x-20 \right) $$$ x = 0 $ is a root of multiplicity $ 3 $.
The second root can be found by solving equation $ 12x-20 = 0$.