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

Terms

a(0) =2a(1) =20a(2) =168a(3) =1320a(4) =10010a(5) =74256a(6) =542640a(7) =3922512a(8) =28120950a(9) =200300100a(10) =1419269280a(11) =10013421600a(12) =70394353848a(13) =493362138080