a(n) = ((6*n+37)*4^n - 1)/3.

Terms

a(0) =12a(1) =57a(2) =261a(3) =1173a(4) =5205a(5) =22869a(6) =99669a(7) =431445a(8) =1856853a(9) =7951701a(10) =33903957a(11) =144004437a(12) =609572181a(13) =2572506453a(14) =10826896725a(15) =45455070549a(16) =190410216789a(17) =796000605525