tn+1 = tn + 5.5 , t1 - 2.5

Find S5, S10, S20

I don't know how to do this

The previous question was -4n+15, n {1, 2, 3...} Find S5, S10, Sn

--> I knew how to do that one but I don't really understand the equation they gave in the second one and what to do with it