3 9 11 17 25 39 ?

Answer 1

3+9 - 1 = 11
9+11 - 2 = 17
11+17 - 3 = 25
17+25 - 4 = 39

ahhh! should your last term be 38 ?
then

term_n = term_(n-1)+term_(n-2) - (n-2) for all n> 2 , term(1) = 3 and term(2) = 9

This is called a recursion formula

If the last term is not 38, I have no idea.