3x²+5xy－2y²﹢x﹢9y－4
=(3x-y)(x+2y)+x+9y-4
=(3x-y)(x+2y)+4x+8y-3x+y-4
=(3x-y)(x+2y)+4(x+2y)-(3x-y)-4
=(3x-y+4)(x+2y-1)