$$ \begin{aligned} 9-\frac{1}{2}x &= 15&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2\cdot9-2 \cdot \frac{1}{2}x &= 2\cdot15&& \text{cancel out the denominators} \\[1 em]18-x &= 30&& \text{ move 18 to the right } \\[1 em]-x &= 30-18&& \\[1 em]-x &= 12&& \text{switch signs on both sides} \\[1 em]x &= -12&& \\[1 em] \end{aligned} $$

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