In order to solve $ \color{blue}{ 3x^{3}-15x^{2}-72x = 0 } $, first we need to factor our $ x $.
$$ 3x^{3}-15x^{2}-72x = x \left( 3x^{2}-15x-72 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ 3x^{2}-15x-72 = 0$.
$ 3x^{2}-15x-72 = 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.