抛物线y^2
y^2-mx-2y+4m+1=0--->(y-1)^2=m(x-1) --->2p=|m|,p/2=|m|/4,顶点M(1+m/4,1),准线x-1=-m/4-->x=1-m/4. 依题意1-m/4=3 --->m=8.