y=-3x²-4x+2
=-3(x²+4/3 x)+2
=-3(x²+4/3 x+4/9-4/9)+2
=-3(x²+4/3 x+4/9)+2+4/3
=-3(x+2/3)²+10/3那y=x²-4x-3配方后是y=（x-2)²-7吗对的