72. $\int e^{3 x} d x$.

The integral of $e^{3x}$ with respect to $x$ is $\frac{1}{3}e^{3x} + C$, where $C$ is the constant of integration. 

However, since the original text only contains the integral expression, I will keep it as is:

72. $\int e^{3 x} d x$.
Solution. According to the previous remark, the exponent should stand under the differential sign, but $d(3 x)=3 d x$. Therefore, it is necessary to add a factor of $1 / 3$. Then we get

$$
\int e^{3 x} d x=\frac{1}{3} \int e^{3 x} d(3 x)=\frac{1}{3} e^{3 x}+C
$$