14 was the right answer
2 8 5 11 14 8 11
2 and 8 have difference of 6
8 and 5 have difference of 3
5 and 11 have difference of 6
11 and 14 has difference of 3
14 and 8 has difference of 6
6 and 1 have difference of 3

so 14 completes the series

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