Question about indefinite integral Let's say I have expression with multiplication which has variable x
$$\int x^2e^{x^3}dx$$
So in example it shows 
$$\int x^2e^{x^3}dx=\frac{1}{3}\int e^u du=\frac{1}{3}e^u+C=\frac{e^{x^3}}{3}+C$$
$u=x^3$ $du=3x^2 dx$
So I don't understand from where comes $\frac{1}{3}$ before integral and from where comes 3 in $du=3x^2 dx$ 

Substituting, you have
$$
\int x^2e^{x^3}dx=\int x^2e^{u}\frac{du}{3x^2}=\frac{1}{3}\int e^u du
$$
after canceling the $x^2$. The $3$ in $du=3x^2 dx$ comes from the fact that the derivative of $x^3$ is $3x^2$ by the power rule.
