Find Recurrence relation

Question

3,5,11,21,43, 85

Answer 1

3,5, 11, 21, 43, 85, 171, ...

an = 2an-1 + (-1)^n-1

the nth term = twice the previous term plus -1 if n is even or +1 if n is odd

2nd term = 2(3) + (-1)^(2-1) = 6-1= 5