$$(x-8)(x+8) = 336$$
Answer
$$ \begin{matrix}x_1 = 20 & x_2 = -20 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (x-8)(x+8) &= 336&& \text{simplify left side} \\[1 em]x^2+8x-8x-64 &= 336&& \\[1 em]x^2+8x-8x-64 &= 336&& \\[1 em]x^2-64 &= 336&& \text{move all terms to the left hand side } \\[1 em]x^2-64-336 &= 0&& \text{simplify left side} \\[1 em]x^2-400 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}-400 = 0 $ is a quadratic equation.
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