$$ \begin{aligned} 7(a+4) &= 20+5a&& \text{simplify left and right hand side} \\[1 em]7a+28 &= 5a+20&& \text{move the $ \color{blue}{ 5a } $ to the left side and $ \color{blue}{ 28 }$ to the right} \\[1 em]7a-5a &= 20-28&& \text{simplify left and right hand side} \\[1 em]2a &= -8&& \text{ divide both sides by $ 2 $ } \\[1 em]a &= -\frac{8}{2}&& \\[1 em]a &= -4&& \\[1 em] \end{aligned} $$

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