Question
$$5x^2+4x^2 = 90$$
Answer
$$ \begin{matrix}x_1 = -\sqrt{ 10 } & x_2 = \sqrt{ 10 } \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} 5x^2+4x^2 &= 90&& \text{simplify left side} \\[1 em]9x^2 &= 90&& \text{move all terms to the left hand side } \\[1 em]9x^2-90 &= 0&& \\[1 em] \end{aligned} $$
$ 9x^{2}-90 = 0 $ is a quadratic equation.
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