2(x＋y)²(x－y)－4(x＋y)³=2（x+y)²（x-y-2x-2y)=2(x+y)²(-x-3y)=-2(x+y)²(x+3y)
25x²(a－b)－b＋a=25x²(a-b)-(a-b)=(a-b)(25x²-1)=(a-b)(5x+1)(5x-1)
6y²＋19y＋15=(2x+3)(3x+5)