3n(n-1)4^(n-2).

Terms

a(0) =0a(1) =0a(2) =6a(3) =72a(4) =576a(5) =3840a(6) =23040a(7) =129024a(8) =688128a(9) =3538944a(10) =17694720a(11) =86507520a(12) =415236096a(13) =1962934272a(14) =9160359936a(15) =42278584320a(16) =193273528320a(17) =876173328384