$$ \begin{aligned} 6+4f &= 6(2f+7)+10f&& \text{simplify left and right hand side} \\[1 em]4f+6 &= 12f+42+10f&& \\[1 em]4f+6 &= 22f+42&& \text{move the $ \color{blue}{ 22f } $ to the left side and $ \color{blue}{ 6 }$ to the right} \\[1 em]4f-22f &= 42-6&& \text{simplify left and right hand side} \\[1 em]-18f &= 36&& \text{ divide both sides by $ -18 $ } \\[1 em]f &= -\frac{36}{18}&& \\[1 em]f &= -2&& \\[1 em] \end{aligned} $$

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