$$ \begin{aligned} 4(2a-8)+7\cdot(-3) &= -(-49a-70)&& \text{simplify left and right hand side} \\[1 em]4(2a-8)+(-21) &= 49a+70&& \\[1 em]8a-32+(-21) &= 49a+70&& \\[1 em]8a-32+(-21) &= 49a+70&& \\[1 em]8a-53 &= 49a+70&& \text{move the $ \color{blue}{ 49a } $ to the left side and $ \color{blue}{ -53 }$ to the right} \\[1 em]8a-49a &= 70+53&& \text{simplify left and right hand side} \\[1 em]-41a &= 123&& \text{ divide both sides by $ -41 $ } \\[1 em]a &= -\frac{123}{41}&& \\[1 em]a &= -3&& \\[1 em] \end{aligned} $$

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