a(n) = (n + 2) * binomial(3*n, n) / (2*n + 1).

Terms

a(0) =2a(1) =3a(2) =12a(3) =60a(4) =330a(5) =1911a(6) =11424a(7) =69768a(8) =432630a(9) =2713425a(10) =17168580a(11) =109390320a(12) =700939512a(13) =4512458580a(14) =29164264320a(15) =189120846288