a(n) = 3*6^n.

Terms

a(0) =3a(1) =18a(2) =108a(3) =648a(4) =3888a(5) =23328a(6) =139968a(7) =839808a(8) =5038848a(9) =30233088a(10) =181398528a(11) =1088391168a(12) =6530347008a(13) =39182082048a(14) =235092492288