a(n) = 1680*(3*n)!/(n!*(2*n + 4)!).

Terms

a(0) =70a(1) =14a(2) =15a(3) =28a(4) =70a(5) =210a(6) =714a(7) =2660a(8) =10626a(9) =44850a(10) =197925a(11) =906192a(12) =4279240a(13) =20746936a(14) =102898110a(15) =520543380a(16) =2679559018a(17) =14007652050a(18) =74240555865a(19) =398363958300