6. The solution to the equation $x=\left(x^{2}-2\right)^{2}-2$ is $\qquad$ _. $(1985$, Shanghai Junior High School Mathematics Competition)
(Let $y=x^{2}-2 . x_{1}=2, x_{2}=-1, x_{3,4}=$
$$
\frac{-1 \pm \sqrt{5}}{2} \text {. })
$$