Answer :
move the pure terms to outside
x^4-9x^2-x^2y^2+9y^2
grop
(x^4-x^2y^2)+(-9x^2+9y^2)
undistribute common factors
(x^2)(x^2-y^2)+(-9)(x^2-y^2)
reverse distributive
(x^2-y^2)(x^2-9)
factor perfect squares
(x-y)(x+y)(x-3)(x+3)
x^4-9x^2-x^2y^2+9y^2
grop
(x^4-x^2y^2)+(-9x^2+9y^2)
undistribute common factors
(x^2)(x^2-y^2)+(-9)(x^2-y^2)
reverse distributive
(x^2-y^2)(x^2-9)
factor perfect squares
(x-y)(x+y)(x-3)(x+3)
