maths coursework sequencesWatch
So you then take the second differences, ie the differences between the first differences: 4, 4, 4...
Since these are 4 all the way up, that indicates that the sequence has something to do with 2n^2.
Subtract each term in the sequence from the corresponding value of 2n^2; this will leave you with a second sequence that ought to be linear. Find an expression for this new sequence, put it together with the 2n^2, and hey presto. Instant formula.
Check that it works for a few values of n, and you're done.
Hope this helps.