$$ \begin{aligned} 3(x-7) &= 5x+9&& \text{simplify left side} \\[1 em]3x-21 &= 5x+9&& \text{move the $ \color{blue}{ 5x } $ to the left side and $ \color{blue}{ -21 }$ to the right} \\[1 em]3x-5x &= 9+21&& \text{simplify left and right hand side} \\[1 em]-2x &= 30&& \text{ divide both sides by $ -2 $ } \\[1 em]x &= -\frac{30}{2}&& \\[1 em]x &= -15&& \\[1 em] \end{aligned} $$

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