(x-2)(x^2-mx+1)
=x^3-mx^2+x-2x^2+2mx-2
=x^3-mx^2-2x^2+2mx+x-2
=x^3-(m+2)x^2+(2m+1)x-2
x的二次项系数为零,
-(m+2)=0
m=-2