3    <--the 1st term
 3 + (-9) = -6    <--the 2nd term
-6 + (18) = 12    <--the 3rd term
12 + (-8) =  4    <--the 4th term
 4 + (16) = 20    <--the 5th term

So using that pattern:

20 + (-7) = 13    <--the 6th trm
13 + (14) = 27    <--the 7th term
27 + (-6) = 21    <--the 8th term
21 + (12) = 33    <--the 9th term
...

Edwin