$$ \begin{aligned} 3x+7 &= 4x-5&& \text{move the $ \color{blue}{ 4x } $ to the left side and $ \color{blue}{ 7 }$ to the right} \\[1 em]3x-4x &= -5-7&& \text{simplify left and right hand side} \\[1 em]-x &= -12&& \text{switch signs on both sides} \\[1 em]x &= 12&& \\[1 em] \end{aligned} $$