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
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The next number in the sequence 3-15-6-30-21 is -207.
28
21
If the last number is 1/3 (it should be), the next number is 5 and 1/3
The sequence is generated using cubes, or n^3. The next number in the sequence will be 4^3, or 256.