4(x-2y)^2-9(2x-y)^2
＝（2(x-2y)+3(2x-y))）(2(x-2y)-3(2x-y)))
=(2x-4y+6x-3y)(2x-4y-6x+3y)
=(8x-7y)(-4x-y)
=(7y-8x)(4x+y)