$$ \begin{aligned} \frac{x}{x+15} &= 9&& \text{multiply ALL terms by } \color{blue}{ x+15 }. \\[1 em](x+15)\frac{x}{x+15} &= (x+15)\cdot9&& \text{cancel out the denominators} \\[1 em]x &= 9x+135&& \text{move the $ \color{blue}{ 9x } $ to the left side} \\[1 em]x-9x &= 135&& \text{simplify left side} \\[1 em]-8x &= 135&& \text{ divide both sides by $ -8 $ } \\[1 em]x &= -\frac{135}{8}&& \\[1 em] \end{aligned} $$

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