(3*6^n + 2^n)/4.

Terms

a(0) =1a(1) =5a(2) =28a(3) =164a(4) =976a(5) =5840a(6) =35008a(7) =209984a(8) =1259776a(9) =7558400a(10) =45349888a(11) =272098304a(12) =1632587776a(13) =9795522560a(14) =58773127168a(15) =352638746624