So stuck on this!
0, 1, 3, 8, 15...... that's the sequence.
I've got (n-1)^2 - 1 as a kind of answer, it's a bit off as it gives -1, 0, 3, 8, 15.
That page lists all the sequences featuring 0, 1, 3, 8, 15 that occur in a very comprehensive list of sequences occurring in mathematics. None of them is given by a simple formula, and there does not seem to a natural formula that generates your sequence.
If I had to guess, I would say that the "1" should not be in the sequence that you were given, and that your suggested answer (where ) is the one that was expected.