$$ \begin{aligned} 7x &= 6+5(3x+3)-x&& \text{simplify right side} \\[1 em]7x &= 6+15x+15-x&& \\[1 em]7x &= 15x+21-x&& \\[1 em]7x &= 14x+21&& \text{move the $ \color{blue}{ 14x } $ to the left side} \\[1 em]7x-14x &= 21&& \text{simplify left side} \\[1 em]-7x &= 21&& \text{ divide both sides by $ -7 $ } \\[1 em]x &= -\frac{21}{7}&& \\[1 em]x &= -3&& \\[1 em] \end{aligned} $$

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