summation - How to determine sum of an alternating power series and to prove that sum is positive

I am working on a problem involving an alternating power series as follows:

$$\sum_{i=0}^{a-2} (-1)^{a+b-i-2}(a+b-i-1)x^{a+b-i-2}$$

$a$ and $b$ is constant with $0

I would like to determine an expression which is used to calculate the sum of this series and to prove the expression is positive. However, I have no idea how to start. Does anyone have any hints or ideas about this problem? Please let me know. I really appreciate your help.