$$ \begin{aligned} x &= x\cdot3-x+3.9x&& \text{simplify right side} \\[1 em]x &= 2x+3.9x&& \\[1 em]x &= 2x+3x&& \\[1 em]x &= 5x&& \text{move the $ \color{blue}{ 5x } $ to the left side} \\[1 em]x-5x &= 0&& \text{simplify left side} \\[1 em]-4x &= 0&& \text{ divide both sides by $ -4 $ } \\[1 em]x &= \frac{0}{-4}&& \\[1 em]x &= 0&& \\[1 em] \end{aligned} $$

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