-3x平方+mx+nx的平方-x+3 = （-3+n)x的平方+(m-1)x+3
值与x的取值无关
-3+n=0,m-1=0
m=1,n=3
-m^2-1/2[3n^2-2(m^2-n^2)]+6
= -m^2-3/2n^2+(m^2-n^2)+6
= -m^2-3/2n^2+m^2-n^2+6
= -5/2n^2+6
= -5/2*3^2+6
= -45/2+6
= -33/2