Well...let's see if we can identify the trend.

511 was released first. Now, use each sequential even number, alternating positive / negative and...

5 + [2] = 7

7 - [4] = 3

3 + [6] = 9

The 9-11 is obviously next <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" /> lol