$$ \begin{aligned} (x+x)\cdot2 &= 3(x+x)+4&& \text{simplify left and right hand side} \\[1 em]2x\cdot2 &= 3\cdot2x+4&& \\[1 em]4x &= 6x+4&& \text{move the $ \color{blue}{ 6x } $ to the left side} \\[1 em]4x-6x &= 4&& \text{simplify left side} \\[1 em]-2x &= 4&& \text{ divide both sides by $ -2 $ } \\[1 em]x &= -\frac{4}{2}&& \\[1 em]x &= -2&& \\[1 em] \end{aligned} $$

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