a(n) = 32*7^n/21 - 8/3, n>=1.

Terms

a(0) =8a(1) =72a(2) =520a(3) =3656a(4) =25608a(5) =179272a(6) =1254920a(7) =8784456a(8) =61491208a(9) =430438472a(10) =3013069320a(11) =21091485256a(12) =147640396808