a(n) = 9*n + 3.

Terms

a(0) =3a(1) =12a(2) =21a(3) =30a(4) =39a(5) =48a(6) =57a(7) =66a(8) =75a(9) =84a(10) =93a(11) =102a(12) =111a(13) =120a(14) =129a(15) =138a(16) =147a(17) =156a(18) =165a(19) =174a(20) =183a(21) =192a(22) =201a(23) =210a(24) =219a(25) =228a(26) =237a(27) =246a(28) =255a(29) =264