m+1=3
m=2
3x^(m-1)-2x^(m+1)-6x^m+3/4
3x-2x^3-6x^2+3/4
=3×(-1/2)+2×（-1/2)^3-6×(-1/2)^2+3/4
=-3/2-1/4-3/2+3/4
=-3+1/2
=-2又1/2