$$ \begin{aligned} f &= -2+\frac{4}{5}f-3&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5f &= -5\cdot2+5 \cdot \frac{4}{5}f-5\cdot3&& \text{cancel out the denominators} \\[1 em]5f &= -10+4f-15&& \text{simplify right side} \\[1 em]5f &= 4f-25&& \text{move the $ \color{blue}{ 4f } $ to the left side} \\[1 em]5f-4f &= -25&& \text{simplify left side} \\[1 em]f &= -25&& \\[1 em]f &= -25&& \\[1 em] \end{aligned} $$

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