a(n) = 6^n - 6.

Terms

a(0) =0a(1) =30a(2) =210a(3) =1290a(4) =7770a(5) =46650a(6) =279930a(7) =1679610a(8) =10077690a(9) =60466170a(10) =362797050a(11) =2176782330a(12) =13060694010a(13) =78364164090a(14) =470184984570