$$ \begin{aligned} 2x+15 &= x+27&& \text{move the $ \color{blue}{ x } $ to the left side and $ \color{blue}{ 15 }$ to the right} \\[1 em]2x-x &= 27-15&& \text{simplify left and right hand side} \\[1 em]x &= 12&& \\[1 em]x &= 12&& \\[1 em] \end{aligned} $$