$$ \begin{aligned} 3 &= -\frac{1}{6}d+1&& \text{multiply ALL terms by } \color{blue}{ 6 }. \\[1 em]6\cdot3 &= -6 \cdot \frac{1}{6}d+6\cdot1&& \text{cancel out the denominators} \\[1 em]18 &= -d+6&& \text{ swap the left and right side of the equation } \\[1 em]-d+6 &= 18&& \text{ move 6 to the right } \\[1 em]-d &= 18-6&& \\[1 em]-d &= 12&& \text{switch signs on both sides} \\[1 em]d &= -12&& \\[1 em] \end{aligned} $$

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