4（a+b)^2(x-y)+9(a-b)^2(y-x)
=4（a+b)^2(x-y)-9(a-b)^2(x-y)
=(x-y)[4（a+b)^2-9(a-b)^2]
=(x-y)[2（a+b)-3(a-b)][2（a+b)+3(a-b)]
=(x-y)(5b-a)(5a-b)