Factor out the constant using $\int af\left(x\right)\mathrm{d}x=a\int f\left(x\right)\mathrm{d}x$.
$$\left(2+1\right)^{2}\int x^{2}\mathrm{d}x$$
Since $\int x^{k}\mathrm{d}x=\frac{x^{k+1}}{k+1}$ for $k\neq -1$, replace $\int x^{2}\mathrm{d}x$ with $\frac{x^{3}}{3}$.
$$\left(2+1\right)^{2}\times \frac{x^{3}}{3}$$
Simplify.
$$3x^{3}$$
If $F\left(x\right)$ is an antiderivative of $f\left(x\right)$, then the set of all antiderivatives of $f\left(x\right)$ is given by $F\left(x\right)+C$. Therefore, add the constant of integration $C\in \mathrm{R}$ to the result.
