11. Let $x \in \mathbf{R}$, for the function $f(x)$ satisfying the condition $f\left(x^{2}+1\right)=x^{4}+5 x^{2}-3$. Then, for all $x \in \mathbf{R}, f\left(x^{2}-1\right)=$ $\qquad$
$$
\text { II. 11. } x^{4}+x^{2}-9 \text {. }
$$

Using the substitution method, we get.
