$$ \begin{aligned} 72 &= 6 \cdot \frac{5+b\cdot2}{2}&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2\cdot72 &= 2\cdot6 \cdot \frac{5+b\cdot2}{2}&& \text{cancel out the denominators} \\[1 em]144 &= 12b+30&& \text{ swap the left and right side of the equation } \\[1 em]12b+30 &= 144&& \text{ move 30 to the right } \\[1 em]12b &= 144-30&& \\[1 em]12b &= 114&& \text{ divide both sides by $ 12 $ } \\[1 em]b &= \frac{114}{12}&& \\[1 em]b &= \frac{19}{2}&& \\[1 em] \end{aligned} $$

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