un = 2n3 - 3n + 5
There may be another way to solve this; however, my method involves the calculus of finite differences and I cannot explain the details here.
What I can tell you is that you should construct a difference table. What you will see is that the 3rd differences are constant. Whenever you see this occur, you will know that the function for which you are looking will be a cubic.
If you want to know more about this solution, you might want to consult the book called "Calculus of Finite Differences" by Jordan.
