$$\frac{n}{2}(3n+1)+\frac{n}{2}(3n+3)$$
$n\left(3n+2\right)$
$$\frac{n\left(3n+1\right)}{2}+\frac{n}{2}\left(3n+3\right)$$
$$\frac{n\left(3n+1\right)}{2}+\frac{n\left(3n+3\right)}{2}$$
$$\frac{n\left(3n+1\right)+n\left(3n+3\right)}{2}$$
$$\frac{3n^{2}+n+3n^{2}+3n}{2}$$
$$\frac{6n^{2}+4n}{2}$$
$$2n+3n^{2}$$
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$3n^{2}+2n$
