2mx^2-x^2+5x+8-(7x^2+5x)
=(2m-1-7)x^2+8
与x无关,则
2m-8=0,m=4
m^2-[2m^2-(5m-4)+m]
=m^2-[2m^2-5m+4+m]
=-m^2+4m-4
=-(m-2)^2
=-(4-2)^2
=-4