The difference increases by one. 3, 6, 10, 15, 21, 28, 36 and so on.
39. The rule here is starting with the number 9, add (-6 + sequence number * 6). 9 + (-6) = 3 3 + 0 = 3 3 + 6 = 9 9 + 12 = 21 21 + 18 = 39 39 + 24 = 63
One possible rule is Un = (-1493n3 + 13443n2 - 38890n + 36024)/6 for n = 1, 2, 3, 4.
27 (-6) = 21
21-28-36-45-55-66-74-87-101