设两个根分别为x1,x2.由韦达定理：x1+x2=m+2；x1*x2=2m^2-1.由上面两个式子可解得：x1^2+x2^2=-3m^2+2m+6.由条件有（x1^2+x2^2）-x1*x2=21,即：-3m^2+2m+6-2m^2+1=21,解得m