a(n) = 9*n^2 - 3.

Terms

a(0) =6a(1) =33a(2) =78a(3) =141a(4) =222a(5) =321a(6) =438a(7) =573a(8) =726a(9) =897a(10) =1086a(11) =1293a(12) =1518a(13) =1761a(14) =2022a(15) =2301a(16) =2598a(17) =2913a(18) =3246a(19) =3597a(20) =3966a(21) =4353a(22) =4758a(23) =5181a(24) =5622a(25) =6081a(26) =6558a(27) =7053a(28) =7566a(29) =8097