a(n) = (12-7*6^n)/5.

Terms

a(0) =1a(1) =-6a(2) =-48a(3) =-300a(4) =-1812a(5) =-10884a(6) =-65316a(7) =-391908a(8) =-2351460a(9) =-14108772a(10) =-84652644a(11) =-507915876a(12) =-3047495268a(13) =-18284971620a(14) =-109709829732a(15) =-658258978404