$$ \begin{aligned} 7\cdot(4+c) &= 6\cdot(2+c)&& \text{simplify left and right hand side} \\[1 em]28+7c &= 12+6c&& \\[1 em]7c+28 &= 6c+12&& \text{move the $ \color{blue}{ 6c } $ to the left side and $ \color{blue}{ 28 }$ to the right} \\[1 em]7c-6c &= 12-28&& \text{simplify left and right hand side} \\[1 em]c &= -16&& \\[1 em]c &= -16&& \\[1 em] \end{aligned} $$

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