$$20+5x-x^2 = 0$$
Answer
$$ \begin{matrix}x_1 = \dfrac{ 5 }{ 2 }-\dfrac{\sqrt{ 105 }}{ 2 } & x_2 = \dfrac{ 5 }{ 2 }+\dfrac{\sqrt{ 105 }}{ 2 } \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} 20+5x-x^2 &= 0&& \text{simplify left side} \\[1 em]-x^2+5x+20 &= 0&& \\[1 em] \end{aligned} $$
$ -x^{2}+5x+20 = 0 $ is a quadratic equation.
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