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

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