Answer
$$\frac{\pi}{12}.$$
Work Step by Step
We have
$$\int_0^3 \frac{dx}{x^2+9}=\frac{1}{3}\int_0^3\frac{d(x/3)}{(x/3)^2+1}=\frac{1}{3}\tan^{-1} (x/3)|_0^3\\
=\frac{1}{3}(\tan^{-1} 1-\tan^{-1}0)=\frac{\pi}{12}.$$
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