a(n) = binomial(3n+6, n).

Terms

a(0) =1a(1) =9a(2) =66a(3) =455a(4) =3060a(5) =20349a(6) =134596a(7) =888030a(8) =5852925a(9) =38567100a(10) =254186856a(11) =1676056044a(12) =11058116888a(13) =73006209045a(14) =482320623240