We have indeterminateness of type
oo/oo,
i.e. limit for the numerator is
$$\lim_{x \to \infty} e^{x} = \infty$$
and limit for the denominator is
$$\lim_{x \to \infty} x^{3} = \infty$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to \infty}\left(\frac{e^{x}}{x^{3}}\right)$$
=
Let's transform the function under the limit a few
$$\lim_{x \to \infty}\left(\frac{e^{x}}{x^{3}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{\frac{d}{d x} e^{x}}{\frac{d}{d x} x^{3}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{x}}{3 x^{2}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{\frac{d}{d x} e^{x}}{\frac{d}{d x} 3 x^{2}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{x}}{6 x}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{\frac{d}{d x} e^{x}}{\frac{d}{d x} 6 x}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{x}}{6}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{x}}{6}\right)$$
=
$$\infty$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 3 time(s)