$$ \begin{aligned} 6a-4(2a+3) &= 7a&& \text{simplify left side} \\[1 em]6a-(8a+12) &= 7a&& \\[1 em]6a-8a-12 &= 7a&& \\[1 em]-2a-12 &= 7a&& \text{move the $ \color{blue}{ 7a } $ to the left side and $ \color{blue}{ -12 }$ to the right} \\[1 em]-2a-7a &= 12&& \text{simplify left side} \\[1 em]-9a &= 12&& \text{ divide both sides by $ -9 $ } \\[1 em]a &= -\frac{12}{9}&& \\[1 em]a &= -\frac{4}{3}&& \\[1 em] \end{aligned} $$

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