4（x+y）²-9（x-y）²
=[2(x+y)]²-[3(x-y)]²
=[2(x+y)+3(x-y)][2(x+y)-3(x-y)]
=(2x+2y+3x-3y)(2x+2y-3x+3y)
=(5x-y)(-x+5y)