$$\frac{\frac{x}{21}}{4} = 3 \cdot \frac{x}{4}+1$$

$$ \begin{aligned} \frac{\frac{x}{21}}{4} &= 3 \cdot \frac{x}{4}+1&& \text{multiply ALL terms by } \color{blue}{ 84 }. \\[1 em]84 \cdot \frac{\frac{x}{21}}{4} &= 84\cdot3 \cdot \frac{x}{4}+84\cdot1&& \text{cancel out the denominators} \\[1 em]x &= 63x+84&& \text{move the $ \color{blue}{ 63x } $ to the left side} \\[1 em]x-63x &= 84&& \text{simplify left side} \\[1 em]-62x &= 84&& \text{ divide both sides by $ -62 $ } \\[1 em]x &= -\frac{84}{62}&& \\[1 em]x &= -\frac{42}{31}&& \\[1 em] \end{aligned} $$

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