5x²+4－3x²+5x－2x²－5+6x
=(5x²－3x²－2x²)+(5x+6x)+(4-5)
=11X-1
当x=-3时.11X-1=11×(-3)-1=-34
5mn－1/2mn²－7/3m²n+3/2mn²－2/3m²n+2m²n
=－7/3m²n－2/3m²n+2m²n+5mn－1/2mn²+3/2mn²
=mn(-7/3m-2/3m+2m+5-1/2n+3/2n)
=mn(-m+5+n)
=-mn(m-5-N)
当m=2,n=-4时,-mn(m-5-N)=-2×(-4)(2-5+4)=8