$$ \begin{aligned} -3(2x+1) &= 3(x+4)&& \text{simplify left and right hand side} \\[1 em]-(6x+3) &= 3x+12&& \\[1 em]-6x-3 &= 3x+12&& \text{move the $ \color{blue}{ 3x } $ to the left side and $ \color{blue}{ -3 }$ to the right} \\[1 em]-6x-3x &= 12+3&& \text{simplify left and right hand side} \\[1 em]-9x &= 15&& \text{ divide both sides by $ -9 $ } \\[1 em]x &= -\frac{15}{9}&& \\[1 em]x &= -\frac{5}{3}&& \\[1 em] \end{aligned} $$

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