$$3 \cdot \frac{n}{4}+3 = 21$$

$$ \begin{aligned} 3 \cdot \frac{n}{4}+3 &= 21&& \text{multiply ALL terms by } \color{blue}{ 4 }. \\[1 em]4\cdot3 \cdot \frac{n}{4}+4\cdot3 &= 4\cdot21&& \text{cancel out the denominators} \\[1 em]3n+12 &= 84&& \text{ move 12 to the right } \\[1 em]3n &= 84-12&& \\[1 em]3n &= 72&& \text{ divide both sides by $ 3 $ } \\[1 em]n &= \frac{72}{3}&& \\[1 em]n &= 24&& \\[1 em] \end{aligned} $$

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