a(n) = ((6*n+19)*4^n - 1)/3.

Terms

a(0) =6a(1) =33a(2) =165a(3) =789a(4) =3669a(5) =16725a(6) =75093a(7) =333141a(8) =1463637a(9) =6378837a(10) =27612501a(11) =118838613a(12) =508908885a(13) =2169853269a(14) =9216283989a(15) =39012619605a(16) =164640413013a(17) =692921390421