$$(x+5)(x-8) = 39$$
Answer
$$ \begin{matrix}x_1 = \dfrac{ 3 }{ 2 }-\dfrac{ 5 \sqrt{ 13}}{ 2 } & x_2 = \dfrac{ 3 }{ 2 }+\dfrac{ 5 \sqrt{ 13}}{ 2 } \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (x+5)(x-8) &= 39&& \text{simplify left side} \\[1 em]x^2-8x+5x-40 &= 39&& \\[1 em]x^2-3x-40 &= 39&& \text{move all terms to the left hand side } \\[1 em]x^2-3x-40-39 &= 0&& \text{simplify left side} \\[1 em]x^2-3x-79 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}-3x-79 = 0 $ is a quadratic equation.
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