a(n) = 3*(5^n-3^n)/2.

Terms

a(0) =0a(1) =3a(2) =24a(3) =147a(4) =816a(5) =4323a(6) =22344a(7) =113907a(8) =576096a(9) =2900163a(10) =14559864a(11) =72976467a(12) =365413776a(13) =1828663203a(14) =9148098984a(15) =45754843827a(16) =228817265856