$$ \begin{aligned} 50-\frac{a}{11} &= 58&& \text{multiply ALL terms by } \color{blue}{ 11 }. \\[1 em]11\cdot50-11 \cdot \frac{a}{11} &= 11\cdot58&& \text{cancel out the denominators} \\[1 em]550-a &= 638&& \text{ move 550 to the right } \\[1 em]-a &= 638-550&& \\[1 em]-a &= 88&& \text{switch signs on both sides} \\[1 em]a &= -88&& \\[1 em] \end{aligned} $$

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