We have indeterminateness of type
oo/-oo,
i.e. limit for the numerator is
$$\lim_{x \to \infty}\left(3 x\right) = \infty$$
and limit for the denominator is
$$\lim_{x \to \infty}\left(3 - x^{2}\right) = -\infty$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to \infty}\left(\frac{3 x}{3 - x^{2}}\right)$$
=
Let's transform the function under the limit a few
$$\lim_{x \to \infty}\left(\frac{3 x}{3 - x^{2}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{\frac{d}{d x} 3 x}{\frac{d}{d x} \left(3 - x^{2}\right)}\right)$$
=
$$\lim_{x \to \infty}\left(- \frac{3}{2 x}\right)$$
=
$$\lim_{x \to \infty}\left(- \frac{3}{2 x}\right)$$
=
$$0$$
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)