方程先化为：f(x)=(x-3)(x+1),所以f[f(x)]=(x^2-2x-6)(x^2-2x-2)=(x-1+(7)^(1/2))(x-1-(7)^(1/2))(x-1+(2)^1/2)(x-1-(3)^(1/2))
f(1-(7)^(1/2))=f(1+(7)^(1/2))=f(1-(2)^1/2))=f(1+(2)^1/2))=0.可以有四个根