a(n) = 6*(10^n-7).

Terms

a(0) =18a(1) =558a(2) =5958a(3) =59958a(4) =599958a(5) =5999958a(6) =59999958a(7) =599999958a(8) =5999999958a(9) =59999999958a(10) =599999999958