∵(2mx²-x²+5x+8)-（7x²-3y+5x)
=2mx²-x²+5x+8-7x²+3y-5x
=2mx²-x²-7x²+3y+8
=2(m-4)x²+3y+8
∴m-4=0,即m=4
则m³-[2m²-（5m-4）+m]
=m³-2m²+5m-4-m
=m³-2m²+4m-4
=4³-2×4²+4×4-4
=64-32+16-4
=44