x²+4x³-2mx²-2x²+nx³-2x+3
=-(1+2m)x²+(4+n)x³-2x+3
若合并同类项后不再出现x的平方和x的立方的项,
则1+2m=0, 4+n=0,
∴m=-1/2, n=-4,
4n²-5mn-7
=4×(-4)²-5×(-1/2)×(-4)-7
=64-10-7
=47