Divide both sides by $f(x)$ and multiply both sides by $dx$:
$$\frac{1}{f(x)} \frac{df}{dx} = 2 dx$$
Now, integrate both sides:
$$\int \frac{1}{f(x)} \frac{df}{dx} dx = \int 2 dx$$
Let $u = f(x)$, then $\frac{du}{dx} = \frac{df}{dx}$. So, we have:
$$\int
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