$$ \begin{aligned} 11+\frac{r}{9} &= 4&& \text{multiply ALL terms by } \color{blue}{ 9 }. \\[1 em]9\cdot11+9 \cdot \frac{r}{9} &= 9\cdot4&& \text{cancel out the denominators} \\[1 em]99+r &= 36&& \text{ move 99 to the right } \\[1 em]r &= 36-99&& \\[1 em]r &= -63&& \\[1 em]r &= -63&& \\[1 em] \end{aligned} $$
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