$$ \begin{aligned} x^2+15^2 &= (9+x)^2&& \text{simplify left and right hand side} \\[1 em]x^2+225 &= 81+18x+x^2&& \\[1 em]x^2+225 &= x^2+18x+81&& \text{move all terms to the left hand side } \\[1 em]x^2+225-x^2-18x-81 &= 0&& \text{simplify left side} \\[1 em]x^2+225-x^2-18x-81 &= 0&& \\[1 em]-18x+144 &= 0&& \text{ move the constants to the right } \\[1 em]-18x &= -144&& \text{ divide both sides by $ -18 $ } \\[1 em]x &= \frac{-144}{-18}&& \\[1 em]x &= 8&& \\[1 em] \end{aligned} $$

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