$$ \begin{aligned} 3+9 &= 3-3(x+7)+2x&& \text{simplify left and right hand side} \\[1 em]12 &= 3-(3x+21)+2x&& \\[1 em]12 &= 3-3x-21+2x&& \\[1 em]12 &= -x-18&& \text{ swap the left and right side of the equation } \\[1 em]-x-18 &= 12&& \text{ move -18 to the right } \\[1 em]-x &= 12+18&& \\[1 em]-x &= 30&& \text{switch signs on both sides} \\[1 em]x &= -30&& \\[1 em] \end{aligned} $$

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