We have indeterminateness of type
-oo/oo,
i.e. limit for the numerator is
$$\lim_{x \to -\infty} x = -\infty$$
and limit for the denominator is
$$\lim_{x \to -\infty} \sqrt{x^{2} + 1} = \infty$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to -\infty}\left(\frac{x}{\sqrt{x^{2} + 1}}\right)$$
=
$$\lim_{x \to -\infty}\left(\frac{\frac{d}{d x} x}{\frac{d}{d x} \sqrt{x^{2} + 1}}\right)$$
=
$$\lim_{x \to -\infty}\left(\frac{\sqrt{x^{2} + 1}}{x}\right)$$
=
$$\lim_{x \to -\infty}\left(\frac{\sqrt{x^{2} + 1}}{x}\right)$$
=
$$-1$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)