The only difficulty here is to decide wether you start counting at 0 or at 1.
a0 = 0-2 = -2
a1 = 3-2 = 1
a2 = 6-2 = 4
a3 = 9-2 = 6
a4 = 12-2 = 10
a5 = 15-2 = 13
So the fifth one is either a4 or a5. Being a computer guy, I tend to start at 0 and choose a4 as the 5th term. The sequence is at 10 then.
