evaluate convergent power series given
$$\sum_{n=0}^\infty x^{n} (n^{2} + n)$$
so using ratio test I have proven that it converges if and only if
$$|x| < 1$$
but I'm not sure how to evaluate this infinite sum.
so I thought about deriving it, but I'm not sure whether I should do it with respect to 
x or n.

$$\begin{align}\sum_{n=0}^\infty x^{n} (n^{2} + n) &=\sum_{n=0}^\infty n(n+1)x^n\\~\\&=\sum_{n=0}^\infty x\left(x^{n+1}\right)''\\~\\&= x \left(\sum_{n=0}^\infty x^{n+1}\right)''\\~\\&=x \left(\dfrac{x}{1-x}\right)''\\~\\&=\dfrac{2x}{(1-x)^3}\end{align}$$
