$$ \begin{aligned} 20-9x &= x+10x&& \text{simplify left and right hand side} \\[1 em]-9x+20 &= 11x&& \text{move the $ \color{blue}{ 11x } $ to the left side and $ \color{blue}{ 20 }$ to the right} \\[1 em]-9x-11x &= -20&& \text{simplify left side} \\[1 em]-20x &= -20&& \text{ divide both sides by $ -20 $ } \\[1 em]x &= \frac{-20}{-20}&& \\[1 em]x &= 1&& \\[1 em] \end{aligned} $$

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