$$ \begin{aligned} -5x+12x+9 &= 4x+42&& \text{simplify left side} \\[1 em]7x+9 &= 4x+42&& \text{move the $ \color{blue}{ 4x } $ to the left side and $ \color{blue}{ 9 }$ to the right} \\[1 em]7x-4x &= 42-9&& \text{simplify left and right hand side} \\[1 em]3x &= 33&& \text{ divide both sides by $ 3 $ } \\[1 em]x &= \frac{33}{3}&& \\[1 em]x &= 11&& \\[1 em] \end{aligned} $$

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