Question 789207
Apply the difference of squares rule (twice) to get 


x^4-81y^4


(x^2)^2-(9y^2)^2


(x^2-9y^2)(x^2+9y^2)


(x^2-(3y)^2)(x^2+9y^2)


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