x四次方-16,
=(x²+4)(x²-4)
=(x²+4)(x+2)(x-2)
a²-3a-ab+3b
=a(a-3)-b(a-3)
=(a-3)(a-b)
,y²-x²+6x-9
=y²-(x-3)²
=(y-x+3)(y+x-3)