TeX source:

\int \frac{1}{9+4 x^{2}} \mathrm{dx}=\frac{1}{2} \int \frac{1}{9+u^{2}} \mathrm{du}=\frac{1}{2} \cdot \frac{1}{3} \arctan \left(\frac{u}{3}\right)+\mathrm{C}=\frac{1}{6} \arctan \left(\frac{2 x}{3}\right)+\mathrm{C} \text {. }