$$-\frac{b}{5}-27 = -21$$

$$ \begin{aligned} -\frac{b}{5}-27 &= -21&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]-5 \cdot \frac{b}{5}-5\cdot27 &= 5\cdot(-21)&& \text{cancel out the denominators} \\[1 em]-b-135 &= -105&& \text{ move -135 to the right } \\[1 em]-b &= -105+135&& \\[1 em]-b &= 30&& \text{switch signs on both sides} \\[1 em]b &= -30&& \\[1 em] \end{aligned} $$

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