a(n) = n*6^n - 1.

Terms

a(0) =5a(1) =71a(2) =647a(3) =5183a(4) =38879a(5) =279935a(6) =1959551a(7) =13436927a(8) =90699263a(9) =604661759a(10) =3990767615a(11) =26121388031a(12) =169789022207