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