Integral of x^2*(x+3) dx

1. Rewrite the integrand:

   $$x^{2} \left(x + 3\right) = x^{3} + 3 x^{2}$$

2. Integrate term-by-term:

   1. The integral of $x^{n}$ is $\frac{x^{n + 1}}{n + 1}$ when $n \neq -1$:

      $$\int x^{3}\, dx = \frac{x^{4}}{4}$$

   1. The integral of a constant times a function is the constant times the integral of the function:

      $$\int 3 x^{2}\, dx = 3 \int x^{2}\, dx$$

      1. The integral of $x^{n}$ is $\frac{x^{n + 1}}{n + 1}$ when $n \neq -1$:

         $$\int x^{2}\, dx = \frac{x^{3}}{3}$$

      So, the result is: $x^{3}$

   The result is: $\frac{x^{4}}{4} + x^{3}$

3. Now simplify:

   $$\frac{x^{3} \left(x + 4\right)}{4}$$

4. Add the constant of integration:

   $$\frac{x^{3} \left(x + 4\right)}{4}+ \mathrm{constant}$$

The answer is:

$$\frac{x^{3} \left(x + 4\right)}{4}+ \mathrm{constant}$$