$$ \begin{aligned} -6a+7\cdot(1+3a) &= 2\cdot(7+8a)&& \text{simplify left and right hand side} \\[1 em]-6a+7+21a &= 14+16a&& \\[1 em]15a+7 &= 16a+14&& \text{move the $ \color{blue}{ 16a } $ to the left side and $ \color{blue}{ 7 }$ to the right} \\[1 em]15a-16a &= 14-7&& \text{simplify left and right hand side} \\[1 em]-a &= 7&& \text{switch signs on both sides} \\[1 em]a &= -7&& \\[1 em] \end{aligned} $$

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