sequences and series - How to compute $sum_{x=0}^{infty}xa^x$?

Need a hint to compute $\displaystyle \sum_{x=0}^\infty xa^x$ and $\displaystyle \sum_{x=0}^\infty x^2a^x$, where $a \in (0,1)$.

Answer

Hint: Differentiate $\sum_{x=0}^{\infty}a^x$ with respect to $a$.