>>558
x^4-4x^3-6x^2+28x-7
= x^4 -6x^2 -7 -4x(x^2 -7)
=(x^2-7)(x^2)+(x^2 -7) -4x(x^2 -7)
=(x^2 -7)(x^2 -4x +1) ≧0

x≦-√7, 2-√3≦x≦√7, 2+√3≦x