(1)2x^2+my-12 -(nx^2-3y+6)
=(2-n)x^2+(m+3)y-18不含x、y
则n=2,m=-3
m+n+mn
=-3+2-2*3
=-7
(2)x^2-x-1=0
==>x^2-x=1
-4x^2+4x+9
=-4(x^2-x)+9
=5