We have indeterminateness of type
oo/oo,
i.e. limit for the numerator is
$$\lim_{n \to \infty}\left(3 \cdot 3^{n} + 1\right) = \infty$$
and limit for the denominator is
$$\lim_{n \to \infty}\left(3^{n} + 1\right) = \infty$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{n \to \infty}\left(\frac{3^{n + 1} + 1}{3^{n} + 1}\right)$$
=
$$\lim_{n \to \infty}\left(\frac{\frac{d}{d n} \left(3 \cdot 3^{n} + 1\right)}{\frac{d}{d n} \left(3^{n} + 1\right)}\right)$$
=
$$\lim_{n \to \infty} 3$$
=
$$\lim_{n \to \infty} 3$$
=
$$3$$
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)