$$3x^3-18x^2 = 21x$$
Answer
$$ \begin{matrix}x_1 = 0 & x_2 = -1 & x_3 = 7 \end{matrix} $$
Explanation
$$ \begin{aligned} 3x^3-18x^2 &= 21x&& \text{move all terms to the left hand side } \\[1 em]3x^3-18x^2-21x &= 0&& \\[1 em] \end{aligned} $$
In order to solve $ \color{blue}{ 3x^{3}-18x^{2}-21x = 0 } $, first we need to factor our $ x $.
$$ 3x^{3}-18x^{2}-21x = x \left( 3x^{2}-18x-21 \right) $$$ x = 0 $ is a root of multiplicity $ 1 $.
The remaining roots can be found by solving equation $ 3x^{2}-18x-21 = 0$.
$ 3x^{2}-18x-21 = 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.
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