a(n) = ((6*n-17)*4^n - 1)/3.

Terms

a(0) =-6a(1) =-15a(2) =-27a(3) =21a(4) =597a(5) =4437a(6) =25941a(7) =136533a(8) =677205a(9) =3233109a(10) =15029589a(11) =68506965a(12) =307582293a(13) =1364546901a(14) =5995058517a(15) =26127717717a(16) =113100805461a(17) =486762960213