$$ \begin{aligned} 12 &= 4-\frac{a}{2}&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2\cdot12 &= 2\cdot4-2 \cdot \frac{a}{2}&& \text{cancel out the denominators} \\[1 em]24 &= 8-a&& \text{ swap the left and right side of the equation } \\[1 em]8-a &= 24&& \text{ move 8 to the right } \\[1 em]-a &= 24-8&& \\[1 em]-a &= 16&& \text{switch signs on both sides} \\[1 em]a &= -16&& \\[1 em] \end{aligned} $$

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