4a^2(x-y)+9(y-x)
=4a^2(x-y)-9(x-y)
=(x-y)(4a^2-9)
=(x-y)(2a+3)(2a-3)
-4x^2y^2+x^3y+4xy^3
=xy(x^2-4xy+4y^2)
=xy(x-2y)^2