Explanation:
I assume the series starts with 7 (and not -7).
in that case I see the following rule :
an = 3×an-1 - 6
7 starting value
15 = 3×7 - 6
39 = 3×15 - 6
111 = 3×39 - 6
the next term would then be
3×111 - 6 = 333 - 6 = 327 (but the series shows 329)
and then the following term would be
3×327 - 6 = 981 - 6 = 975
so, all fits except for 329.
so, 329 is x the wrong term. it should have been 327.