9(2x - y)^2 - 4(x + 2y)^2
=(6x - 3y)^2 - (2x + 4y)^2
=(6x - 3y + 2x + 4y)(6x - 3y - 2x - 4y)
=(8x + y)(4x - 7y)
希望能帮你!