$$7 = \frac{d-1}{12}$$

$$ \begin{aligned} 7 &= \frac{d-1}{12}&& \text{multiply ALL terms by } \color{blue}{ 12 }. \\[1 em]12\cdot7 &= 12 \cdot \frac{d-1}{12}&& \text{cancel out the denominators} \\[1 em]84 &= d-1&& \text{ swap the left and right side of the equation } \\[1 em]d-1 &= 84&& \text{ move -1 to the right } \\[1 em]d &= 84+1&& \\[1 em]d &= 85&& \\[1 em]d &= 85&& \\[1 em] \end{aligned} $$

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