a(n) = (11*7^n + 1)/2.

Terms

a(0) =6a(1) =39a(2) =270a(3) =1887a(4) =13206a(5) =92439a(6) =647070a(7) =4529487a(8) =31706406a(9) =221944839a(10) =1553613870a(11) =10875297087a(12) =76127079606a(13) =532889557239