3 ((-9)) -6 ((+18)) 12 ((-8)) 4 ((+16)) 20
There are many possibilities- you can always make a more complicated equation to fit any length of starting numbers, but the simplest answer looks like:
See the -9, -8, etc., and the +18, +16, etc.? Extend that pattern with -7, -6, ... and +14, +12, ... .
3 ((-9)) -6 ((+18)) 12 ((-8)) 4 ((+16)) 20 ((-7)) 13 ((+14)) 27 ((-6)) 21 ((+12)) 33
So the sequence would be:
3, -6, 12, 4, 20, 13, 27, 21, 33
The next number would be:
13