-1+m-3n+m+n-1=0
m=n+1
x^3＋mx^2＋3nx＋m＋n-1
=x^3+(n+1)x^2+3nx+2n
=(x+1)(x^2+nx+2n)=0

x^2+nx+2n=0の判別式D=n^2-4*2n=n(n-8)≧0
n≦0,8≦n