Answer
$$f=\frac{1}{2X_C \pi C} .$$
Work Step by Step
To solve the equation for $f$, we multiply both sides by $2\pi f C$ and then divide both sides by $2\pi CX_C$ as follows: $$X_C=\frac{1}{2\pi fC} \Longrightarrow2X_C f\pi C=1\Longrightarrow \frac{2X_C f\pi C}{2X_C \pi C}=\frac{1}{2X_C \pi C}\\ \Longrightarrow f=\frac{1}{2X_C \pi C} .$$
