多项式（2mx2次方-x2次方+3x+1）—(5x2次方-4y+3x）的值与x无关
=2mx^2-x^2+3x+1-5x^2+4y-3x
=(2m-6)x^2+1+4y
2m-6=0
m=3
2m的二次方—[3m的2次方+（4m-5）+m]
=2m^2-[3m^2+5m-5]
=2m^2-3m^2-5m+5
=-m^2-5m+5
=-3^2-5*3+5
=-9-15+5
=-19