I'll leave some of the details between the equalities, as well as the final step, to you.
I totally screwed this problem up earlier. I understand what needs to be done but it's been 35 years since I studied sequences and series. Could someone help by giving me a hint on how to proceed?
I need to factor out np from the following sigma summation:
Now factoring out the np is not a problem in itself, and I need to change the limits by letting y=x-1 so I can sum over y=0 to y=n-1, but what do I do with the in the first term?