a(n) = 6n + 3^n.

Terms

a(0) =9a(1) =21a(2) =45a(3) =105a(4) =273a(5) =765a(6) =2229a(7) =6609a(8) =19737a(9) =59109a(10) =177213a(11) =531513a(12) =1594401a(13) =4783053a(14) =14348997a(15) =43046817a(16) =129140265a(17) =387420597a(18) =1162261581a(19) =3486784521a(20) =10460353329a(21) =31381059741a(22) =94143178965a(23) =282429536625a(24) =847288609593