a(n) = (4*n+1)*binomial(3*n,n)/(2*n+1).

Terms

a(0) =1a(1) =5a(2) =27a(3) =156a(4) =935a(5) =5733a(6) =35700a(7) =224808a(8) =1427679a(9) =9126975a(10) =58659315a(11) =378658800a(12) =2453288292a(13) =15944020316a(14) =103897691640a(15) =678610095504