$$ \begin{aligned} -7x-(x+6) &= 9+2x&& \text{simplify left and right hand side} \\[1 em]-7x-x-6 &= 2x+9&& \\[1 em]-8x-6 &= 2x+9&& \text{move the $ \color{blue}{ 2x } $ to the left side and $ \color{blue}{ -6 }$ to the right} \\[1 em]-8x-2x &= 9+6&& \text{simplify left and right hand side} \\[1 em]-10x &= 15&& \text{ divide both sides by $ -10 $ } \\[1 em]x &= -\frac{15}{10}&& \\[1 em]x &= -\frac{3}{2}&& \\[1 em] \end{aligned} $$

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