Question 663285
f(x) = x^5 - 81x


f(x) = x(x^4 - 81)


f(x) = x( (x^2)^2 - 9^2 )


f(x) = x(x^2 - 9)(x^2 + 9)


f(x) = x(x^2 - 3^2)(x^2 + 9)


f(x) = <font color="red">x(x - 3)(x + 3)(x^2 + 9)</font>