$$ \begin{aligned} \frac{1}{2}x-\frac{2}{7}x &= 3&& \text{multiply ALL terms by } \color{blue}{ 14 }. \\[1 em]14 \cdot \frac{1}{2}x-14\frac{2}{7}x &= 14\cdot3&& \text{cancel out the denominators} \\[1 em]7x-4x &= 42&& \text{simplify left side} \\[1 em]3x &= 42&& \text{ divide both sides by $ 3 $ } \\[1 em]x &= \frac{42}{3}&& \\[1 em]x &= 14&& \\[1 em] \end{aligned} $$

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