The plain old boring way:

t1 = 2.5

t2 = t1 + 5.5 = 2.5 + 5.5 = 8

t3 = t2 + 5.5 = 8 + 5.5 = 13.5

etc.

Or you could note that tn+1 - tn = 5.5 and so there's the same difference between two successive terms. So you have an arithmetic series:

First term = 2.5

Common difference = 5.5

Therefore tn = 2.5 + (n-1)(5.5) = ....