a(n) = (n+1)*(n+6)*3^n/6.

Terms

a(0) =1a(1) =7a(2) =36a(3) =162a(4) =675a(5) =2673a(6) =10206a(7) =37908a(8) =137781a(9) =492075a(10) =1732104a(11) =6022998a(12) =20726199a(13) =70681653a(14) =239148450a(15) =803538792a(16) =2683245609a(17) =8910671247a(18) =29443957164a(19) =96855122250a(20) =317297380491