Question
$$b(b-1) = 90$$
Answer
$$ \begin{matrix}b_1 = -9 & b_2 = 10 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} b(b-1) &= 90&& \text{simplify left side} \\[1 em]b^2-b &= 90&& \text{move all terms to the left hand side } \\[1 em]b^2-b-90 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}-x-90 = 0 $ is a quadratic equation.
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