a(n) = 3*n^2 + 12*n.

Terms

a(0) =15a(1) =36a(2) =63a(3) =96a(4) =135a(5) =180a(6) =231a(7) =288a(8) =351a(9) =420a(10) =495a(11) =576a(12) =663a(13) =756a(14) =855a(15) =960a(16) =1071a(17) =1188a(18) =1311a(19) =1440a(20) =1575a(21) =1716a(22) =1863a(23) =2016a(24) =2175a(25) =2340a(26) =2511a(27) =2688a(28) =2871a(29) =3060