$$ \begin{aligned} -2x+1+2 &= x+3x+21&& \text{simplify left and right hand side} \\[1 em]-2x+3 &= 4x+21&& \text{move the $ \color{blue}{ 4x } $ to the left side and $ \color{blue}{ 3 }$ to the right} \\[1 em]-2x-4x &= 21-3&& \text{simplify left and right hand side} \\[1 em]-6x &= 18&& \text{ divide both sides by $ -6 $ } \\[1 em]x &= -\frac{18}{6}&& \\[1 em]x &= -3&& \\[1 em] \end{aligned} $$

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