$$ \begin{aligned} \frac{7-x}{4} &= \frac{1}{3}&& \text{multiply ALL terms by } \color{blue}{ 12 }. \\[1 em]12 \cdot \frac{7-x}{4} &= 12\cdot\frac{1}{3}&& \text{cancel out the denominators} \\[1 em]-3x+21 &= 4&& \text{ move 21 to the right } \\[1 em]-3x &= 4-21&& \\[1 em]-3x &= -17&& \text{ divide both sides by $ -3 $ } \\[1 em]x &= \frac{-17}{-3}&& \\[1 em]x &= \frac{17}{3}&& \\[1 em] \end{aligned} $$

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