4A-2B
=4(nx²+3mx+5)-2(4x²-3x-y²)
=4nx²+12mx+20-8x²+6x+2y²
=(4n-8)x²+(12m+6)x+2y²+20
与字母x无关,
则,4n-8=0,12m+6=0
所以,n=2,m=-1/2