4-12（m-n)+9(m-n)^2
=[3(m-n)-2]^2
=(3m-3n-2)^2
(x^2+4y^2)^2-16x^2y^2
=(x^2+4y^2+4xy)(x^2+4y^2-4xy)
=(x+2y)^2(x-2y)^2