$$ \begin{aligned} 4-\frac{f}{8} &= 7&& \text{multiply ALL terms by } \color{blue}{ 8 }. \\[1 em]8\cdot4-8 \cdot \frac{f}{8} &= 8\cdot7&& \text{cancel out the denominators} \\[1 em]32-f &= 56&& \text{ move 32 to the right } \\[1 em]-f &= 56-32&& \\[1 em]-f &= 24&& \text{switch signs on both sides} \\[1 em]f &= -24&& \\[1 em] \end{aligned} $$

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