The problem can be simplified if u take out the odd numbered terms (1st, 3rd, 5th term and so on). This leaves the pattern an obvious one, -11 from the previous number. Since we took out half the numbers, 56th term will be 28th term in the new pattern. There is an actual formula for this arithmetic progression.
Un= U + (n-1)(D), but there is no need to memorise, it's just for your info.