3x^2+my-8-(-nx^2+2y+7)
=3x^2+my-8+nx^2-2y-7
=3x^2+nx^2+my-2y-8-7
=(3+n)x^2+(m-2)y-15
不含有x,y项
即3+n=0,m-2=0
m=2
n=-3
n^m+mn
=(-3)^2+2*(-3)
=9-6
=3
你好,很高兴为你解答,希望对你有所帮助,若满意请采纳.