#### Answer

$\color{blue}{a+b=\dfrac{f}{c^2}}$

#### Work Step by Step

Use the rule $\frac{a}{b}=\frac{c}{d}\longrightarrow ad=bc$ to obtain:
$$c\cdot (a+b)c=f
\\c^2(a+b)=f$$
Divide $c^2$ to both sides to obtain:
$$\frac{c^2(a+b)}{c^2}=\frac{f}{c^2}
\\\color{blue}{a+b=\frac{f}{c^2}}$$