Answer:
( x + 3)( x - 3)( x + y )( x - y )
Step-by-step explanation:
You can group the terms in pairs and factor each pair:
-x^2y^2 + x^4 + 9y^2 - 9x^2 = x^2(-y^2+x^2) -9(-y^2+x^2)
= (x^2 -9)(x^2 -y^2)
Each of these factors is the difference of squares, so can be factored as such:
= (x +3)(x -3)(x +y)(x -y)
