a(n) = n^2 * binomial(3*n, n).

Terms

a(0) =0a(1) =3a(2) =60a(3) =756a(4) =7920a(5) =75075a(6) =668304a(7) =5697720a(8) =47070144a(9) =379632825a(10) =3004501500a(11) =23417943120a(12) =180241588800