3^(n-1)*(2*3^(n-1)+3)+1.

Terms

a(0) =6a(1) =28a(2) =190a(3) =1540a(4) =13366a(5) =118828a(6) =1065070a(7) =9572500a(8) =86113126a(9) =774900028a(10) =6973745950a(11) =62762650660a(12) =564860667286