$$ \begin{aligned} 3x+3 &= 6x-9&& \text{move the $ \color{blue}{ 6x } $ to the left side and $ \color{blue}{ 3 }$ to the right} \\[1 em]3x-6x &= -9-3&& \text{simplify left and right hand side} \\[1 em]-3x &= -12&& \text{ divide both sides by $ -3 $ } \\[1 em]x &= \frac{-12}{-3}&& \\[1 em]x &= 4&& \\[1 em] \end{aligned} $$
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