$$ \begin{aligned} 6(3a+1)-30 &= 3(2a-4)&& \text{simplify left and right hand side} \\[1 em]18a+6-30 &= 6a-12&& \\[1 em]18a-24 &= 6a-12&& \text{move the $ \color{blue}{ 6a } $ to the left side and $ \color{blue}{ -24 }$ to the right} \\[1 em]18a-6a &= -12+24&& \text{simplify left and right hand side} \\[1 em]12a &= 12&& \text{ divide both sides by $ 12 $ } \\[1 em]a &= \frac{12}{12}&& \\[1 em]a &= 1&& \\[1 em] \end{aligned} $$

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