$$\frac{1}{3}(9x-42) = 2x+12$$

$$ \begin{aligned} \frac{1}{3}(9x-42) &= 2x+12&& \text{simplify left side} \\[1 em]\frac{9x-42}{3} &= 2x+12&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3 \cdot \frac{9x-42}{3} &= 3\cdot2x+3\cdot12&& \text{cancel out the denominators} \\[1 em]9x-42 &= 6x+36&& \text{move the $ \color{blue}{ 6x } $ to the left side and $ \color{blue}{ -42 }$ to the right} \\[1 em]9x-6x &= 36+42&& \text{simplify left and right hand side} \\[1 em]3x &= 78&& \text{ divide both sides by $ 3 $ } \\[1 em]x &= \frac{78}{3}&& \\[1 em]x &= 26&& \\[1 em] \end{aligned} $$

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