a(n) = (7*3^n + 2n + 5)/4.

Terms

a(0) =3a(1) =7a(2) =18a(3) =50a(4) =145a(5) =429a(6) =1280a(7) =3832a(8) =11487a(9) =34451a(10) =103342a(11) =310014a(12) =930029a(13) =2790073a(14) =8370204a(15) =25110596a(16) =75331771a(17) =225995295a(18) =677985866a(19) =2033957578a(20) =6101872713a(21) =18305618117a(22) =54916854328a(23) =164750562960a(24) =494251688855