$$ \begin{aligned} -\frac{b}{5}-27 &= -21\cdot0&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]-5 \cdot \frac{b}{5}-5\cdot27 &= -5\cdot21\cdot0&& \text{cancel out the denominators} \\[1 em]-b-135 &= -0&& \text{move the $ \color{blue}{ 0 } $ to the left side and $ \color{blue}{ -135 }$ to the right} \\[1 em]-b+0 &= 135&& \text{switch signs on both sides} \\[1 em]b &= -135&& \\[1 em] \end{aligned} $$

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