I first posted my question in the wrong forum (I did not know that there was an economic forum now) so I want to apologise for that by User Ben Bolker and User Robert. Also special thanks to User Robert who gave me the information with the econonomics forum!
So my question is about the Samuelson acceleration model. I am trying to help a friend of mine solve this question. I am not an economic studied person. I attached the question and my answers and conclusions. Maybe some of you can give me a little bit of help. In the first case, I did interpret the whole topic wrong and was confused because of the difference and thought this should be possible with an derivation (which was absolutely wrong, in my opionion now). I found an old analysis economics book and there were some interesting facts about the question (I also attached these facts).
The main question is, is it possible to bring the function back to the standard form of an inhomogen linear style with index inversion or something else. You can see what I have done attached, but I could not change the following from t-1 to t+1 because I did not know how.
Maybe some specialists here can help me.