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

Terms

a(0) =280a(1) =210a(2) =420a(3) =1176a(4) =3960a(5) =15015a(6) =61880a(7) =271320a(8) =1248072a(9) =5965050a(10) =29414700a(11) =148874400a(12) =770263200a(13) =4061212722a(14) =21765976680a(15) =118336861720a(16) =651555929640