$$ \begin{aligned} 15x-27 &= (6+x)\cdot6&& \text{simplify right side} \\[1 em]15x-27 &= 36+6x&& \\[1 em]15x-27 &= 6x+36&& \text{move the $ \color{blue}{ 6x } $ to the left side and $ \color{blue}{ -27 }$ to the right} \\[1 em]15x-6x &= 36+27&& \text{simplify left and right hand side} \\[1 em]9x &= 63&& \text{ divide both sides by $ 9 $ } \\[1 em]x &= \frac{63}{9}&& \\[1 em]x &= 7&& \\[1 em] \end{aligned} $$

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