m^2x-9n^2x-9n^2y+m^2y
= m^2x+m^2y - (9n^2x+9n^2y )
= m^2 (x+y) - 9n^2 (x+y)
= (m^2 - 9n^2 ) (x+y)
= (m+3n ) (m-3n ) (x+y )