$$ \begin{aligned} 3x\cdot5 &= 3(5x-4)-12x&& \text{simplify left and right hand side} \\[1 em]15x &= 15x-12-12x&& \\[1 em]15x &= 3x-12&& \text{move the $ \color{blue}{ 3x } $ to the left side} \\[1 em]15x-3x &= -12&& \text{simplify left side} \\[1 em]12x &= -12&& \text{ divide both sides by $ 12 $ } \\[1 em]x &= -\frac{12}{12}&& \\[1 em]x &= -1&& \\[1 em] \end{aligned} $$

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