Question
$$(20x-21)^2 = 81$$
Answer
$$ \begin{matrix}x_1 = \dfrac{ 3 }{ 2 } & x_2 = \dfrac{ 3 }{ 5 } \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (20x-21)^2 &= 81&& \text{simplify left side} \\[1 em]400x^2-840x+441 &= 81&& \text{move all terms to the left hand side } \\[1 em]400x^2-840x+441-81 &= 0&& \text{simplify left side} \\[1 em]400x^2-840x+360 &= 0&& \\[1 em] \end{aligned} $$
$ 400x^{2}-840x+360 = 0 $ is a quadratic equation.
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