Example 2. Find $\int 3^{7 x-1 / 9} d x$.
Solution. From formula 4 of the table, when $u=7 x-1 / 9$, we get

$$
\int 3^{7 x-1 / 9} d x=\frac{1}{7 \cdot \ln 3} 3^{7 x-1 / 9}+C
$$

Note. In the future, to ensure the continuity of integration, auxiliary transformations, notations, and remarks will be enclosed in curly braces throughout the solution.