a(n) = 8*3^n - 12.

Terms

a(0) =12a(1) =60a(2) =204a(3) =636a(4) =1932a(5) =5820a(6) =17484a(7) =52476a(8) =157452a(9) =472380a(10) =1417164a(11) =4251516a(12) =12754572a(13) =38263740a(14) =114791244a(15) =344373756a(16) =1033121292a(17) =3099363900a(18) =9298091724a(19) =27894275196a(20) =83682825612a(21) =251048476860a(22) =753145430604