a(n) = (3 - 6*n)*(-1)^n.

Terms

a(0) =0a(1) =3a(2) =-9a(3) =15a(4) =-21a(5) =27a(6) =-33a(7) =39a(8) =-45a(9) =51a(10) =-57a(11) =63a(12) =-69a(13) =75a(14) =-81a(15) =87a(16) =-93a(17) =99a(18) =-105a(19) =111a(20) =-117a(21) =123a(22) =-129a(23) =135a(24) =-141a(25) =147a(26) =-153a(27) =159a(28) =-165a(29) =171