a(n) = 2*binomial(3*n,n) - 9*binomial(2*n,n).

Terms

a(0) =-7a(1) =-12a(2) =-24a(3) =-12a(4) =360a(5) =3738a(6) =28812a(7) =201672a(8) =1355112a(9) =8936070a(10) =58427226a(11) =380724552a(12) =2479017996a(13) =16151245488a(14) =105359408760a(15) =688338793488