(3a^(n+1)-6a^n+9a^(n-1)/(1/3a^(n-1)
=3[3a^(n+1)-6a^n+9a^(n-1)]/a^(n-1)
=3[3an^(n-1)(a^2-2a+3)]/a^(n-1)
=3*3*(a^2-2a+3)
=9a²-18a+27