In order to solve $ \color{blue}{ -24x^{9}+42x^{8}+48x^{7} = 0 } $, first we need to factor our $ x^7 $.
$$ -24x^{9}+42x^{8}+48x^{7} = x^7 \left( -24x^{2}+42x+48 \right) $$$ x = 0 $ is a root of multiplicity $ 7 $.
The remaining roots can be found by solving equation $ -24x^{2}+42x+48 = 0$.
$ -24x^{2}+42x+48 = 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.