You can put this solution on YOUR website! First let us factor out an "a" as our common factor, so that
b^4a - 81a^5 =
a(b^4 - 81a^4) =
Now we have the difference of squares, and that factors out to
a(b^2 + 9a^2)(b^2 - 9a^2) =
But this last term factors out once more as the difference of squares, and we have
a(b^2 + 9a^2)(b + 3a)(b - 3a)
