$$ \begin{aligned} 18 &= 6x+6(x-7)&& \text{simplify right side} \\[1 em]18 &= 6x+6x-42&& \\[1 em]18 &= 12x-42&& \text{ swap the left and right side of the equation } \\[1 em]12x-42 &= 18&& \text{ move -42 to the right } \\[1 em]12x &= 18+42&& \\[1 em]12x &= 60&& \text{ divide both sides by $ 12 $ } \\[1 em]x &= \frac{60}{12}&& \\[1 em]x &= 5&& \\[1 em] \end{aligned} $$

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