a(n) = (3*n+3)!/(3*n!*(2*n+2)!).

Terms

a(0) =1a(1) =10a(2) =84a(3) =660a(4) =5005a(5) =37128a(6) =271320a(7) =1961256a(8) =14060475a(9) =100150050a(10) =709634640a(11) =5006710800a(12) =35197176924a(13) =246681069040