When I tried solving it myself I got kind of stuck at $(2a-1)^n * (2a-1) - 1$, as I haven't done any induction proof myself in a quite a while. Any help is appreciated!
PS: Sorry if my formatting is wrong, this is my first post on math stackexchange.
PPS: If anyone has seen this question before the edit: I have added that the formula has to be equal to an even number, which I forgot to say before, my apologies.
3=2, I can prove something about it, but if I say3there's nothing to prove. – NickD Dec 05 '22 at 17:51