$$(x-4)\frac{x+5}{2} = 0$$
Answer
$$ \begin{matrix}x_1 = 4 & x_2 = -5 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (x-4)\frac{x+5}{2} &= 0&& \text{simplify left side} \\[1 em]\frac{x^2+x-20}{2} &= 0&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2 \cdot \frac{x^2+x-20}{2} &= 2\cdot0&& \text{cancel out the denominators} \\[1 em]x^2+x-20 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}+x-20 = 0 $ is a quadratic equation.
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