In order to solve $ \color{blue}{ 4x^{3}+8x^{2}-140x = 0 } $, first we need to factor our $ x $.
$$ 4x^{3}+8x^{2}-140x = x \left( 4x^{2}+8x-140 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ 4x^{2}+8x-140 = 0$.
$ 4x^{2}+8x-140 = 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.