$$ \begin{aligned} 21+x &= \frac{29}{5}&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5\cdot21+5x &= 5\cdot\frac{29}{5}&& \text{cancel out the denominators} \\[1 em]105+5x &= 29&& \text{ move 105 to the right } \\[1 em]5x &= 29-105&& \\[1 em]5x &= -76&& \text{ divide both sides by $ 5 $ } \\[1 em]x &= -\frac{76}{5}&& \\[1 em] \end{aligned} $$

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