Question
$$x(x-4) = 21$$
Answer
$$ \begin{matrix}x_1 = -3 & x_2 = 7 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x(x-4) &= 21&& \text{simplify left side} \\[1 em]x^2-4x &= 21&& \text{move all terms to the left hand side } \\[1 em]x^2-4x-21 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}-4x-21 = 0 $ is a quadratic equation.
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