I need help with questions 3 and 4. For #3, I solved everything except the explicit expression. I don't understand #4 at all. It doesn't seem to be linear, exponential, or quadratic. For #3, I got f(x)=2x+4 as the explicit expression. This however only solves for the first common difference. I would appreciate help solving this.

For #4, I had no clue what to do, so I put random stuff. I would appreciate help fixing it.

Thanks!