$$ \begin{aligned} 8-(2a+7) &= a+40&& \text{simplify left side} \\[1 em]8-2a-7 &= a+40&& \\[1 em]-2a+1 &= a+40&& \text{move the $ \color{blue}{ a } $ to the left side and $ \color{blue}{ 1 }$ to the right} \\[1 em]-2a-a &= 40-1&& \text{simplify left and right hand side} \\[1 em]-3a &= 39&& \text{ divide both sides by $ -3 $ } \\[1 em]a &= -\frac{39}{3}&& \\[1 em]a &= -13&& \\[1 em] \end{aligned} $$

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