a(n) = 52*7^n/21 - 16/3 (n>=1).

Terms

a(0) =12a(1) =116a(2) =844a(3) =5940a(4) =41612a(5) =291316a(6) =2039244a(7) =14274740a(8) =99923212a(9) =699462516a(10) =4896237644a(11) =34273663540a(12) =239915644812