$$ \begin{aligned} -13 &= \frac{r}{9}+8&& \text{multiply ALL terms by } \color{blue}{ 9 }. \\[1 em]9\cdot(-13) &= 9 \cdot \frac{r}{9}+9\cdot8&& \text{cancel out the denominators} \\[1 em]-117 &= r+72&& \text{ swap the left and right side of the equation } \\[1 em]r+72 &= -117&& \text{ move 72 to the right } \\[1 em]r &= -117-72&& \\[1 em]r &= -189&& \\[1 em]r &= -189&& \\[1 em] \end{aligned} $$

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