$$ \begin{aligned} \frac{21x}{5}+7x &= 0&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5 \cdot \frac{21x}{5}+5\cdot7x &= 5\cdot0&& \text{cancel out the denominators} \\[1 em]21x+35x &= 0&& \text{simplify left side} \\[1 em]56x &= 0&& \text{ divide both sides by $ 56 $ } \\[1 em]x &= \frac{0}{56}&& \\[1 em]x &= 0&& \\[1 em] \end{aligned} $$

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