①x²-8x+12
=(x-6)(x-2)
②a²+3ab-4b²
=(a+4b)(a-b)
③a²-b²-2b-1
=a²-(b+1)²
=(a-b-1)(a+b+1)
④4-(3-x)²
=[2-(3-x)][2+(3-x)]
=(x-1)(5-x)