(2m-1)²-m²b²-4m²b+4m²=(2m-1)²-m²(b²-4b+4)=(2m-1)²-m²(b-2)²=[(2m-1)+m(b-2)]×[(2m-1)-m(b-2)]=(mb-1)(4m-mb+2m-1)=(mb-1)(6m-mb-1)