a(n) = 420*(3*n)!/(n!*(2*n + 3)!).

Terms

a(0) =70a(1) =21a(2) =30a(3) =70a(4) =210a(5) =735a(6) =2856a(7) =11970a(8) =53130a(9) =246675a(10) =1187550a(11) =5890248a(12) =29954680a(13) =155602020a(14) =823184880a(15) =4424618730a(16) =24116031162a(17) =133072694475a(18) =742405558650