$$ \begin{aligned} \frac{3}{7} &= \frac{3}{4}-\frac{x}{2}&& \text{multiply ALL terms by } \color{blue}{ 28 }. \\[1 em]28\cdot\frac{3}{7} &= 28\cdot\frac{3}{4}-28\frac{x}{2}&& \text{cancel out the denominators} \\[1 em]12 &= 21-14x&& \text{ swap the left and right side of the equation } \\[1 em]21-14x &= 12&& \text{ move 21 to the right } \\[1 em]-14x &= 12-21&& \\[1 em]-14x &= -9&& \text{ divide both sides by $ -14 $ } \\[1 em]x &= \frac{-9}{-14}&& \\[1 em]x &= \frac{9}{14}&& \\[1 em] \end{aligned} $$

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