Answer
$$f=\frac{X_L}{2\pi L}.$$
Work Step by Step
To solve the given equation for $f$, we divide both sides by $2\pi L$ as follows
$$\frac{X_L}{2\pi L}=\frac{2\pi L f}{2\pi L}\Longrightarrow f=\frac{X_L}{2\pi L}.$$
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