a(n) = 3*(9^n - 1)/4.

Terms

a(0) =0a(1) =6a(2) =60a(3) =546a(4) =4920a(5) =44286a(6) =398580a(7) =3587226a(8) =32285040a(9) =290565366a(10) =2615088300a(11) =23535794706a(12) =211822152360