方程写成一般式为：x²-2(m-3)x+m²-4m+3=0
降次：
x1,x2是该方程的根,则：x1+x2=2(m-3),x1x2=m²-4m+3
且：x1²-2(m-3)x1+m²-4m+3=0,x2²-2(m-3)x2+m²-4m+3=0
得：x1²=2(m-3)x1-m²+4m-3,x2²=2(m-3)x2-m²+4m-3
则：x1²+x2²=2(m-3)(x1+x2)-2m²+8m-6
所以,x1x2-x1²-x2²=x1x2-2(m-3)(x1+x2)+2m²-8m+6
=m²-4m+3-4(m-3)²+2m²-8m+6
=-m²+12m-27
=-(m-6)²+9