(3x+my-8)-(-nx+2y+7)=(3x+nx)+(my-2y)+(-8-7),
不含x、y项,说明：(3x+nx)=0,(my-2y)=0,解得：n=-3; m=2.
n的m次方+mn
=(-3)²+2*(-3)
=9-6
=3