$$ \begin{aligned} 3 &= -2(f+4)-13&& \text{simplify right side} \\[1 em]3 &= -(2f+8)-13&& \\[1 em]3 &= -2f-8-13&& \\[1 em]3 &= -2f-21&& \text{ swap the left and right side of the equation } \\[1 em]-2f-21 &= 3&& \text{ move -21 to the right } \\[1 em]-2f &= 3+21&& \\[1 em]-2f &= 24&& \text{ divide both sides by $ -2 $ } \\[1 em]f &= -\frac{24}{2}&& \\[1 em]f &= -12&& \\[1 em] \end{aligned} $$

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