4x²-m²-y²+2ym
=4x²-m²+2ym-y²
=4x²-(m²-2ym+y²)
=4x²-(m-y)²
=(2x+m-y)(2x-m+y)
（x²-1）²-6（x²-1）+9
=(x²-1-3)²
=(x²-4)²
=(x+2)²(x-2)²