a(n) = 9*3^n - 15.

Terms

a(0) =12a(1) =66a(2) =228a(3) =714a(4) =2172a(5) =6546a(6) =19668a(7) =59034a(8) =177132a(9) =531426a(10) =1594308a(11) =4782954a(12) =14348892a(13) =43046706a(14) =129140148a(15) =387420474a(16) =1162261452a(17) =3486784386a(18) =10460353188a(19) =31381059594a(20) =94143178812a(21) =282429536466a(22) =847288609428