Detail solution
Let's take the limit
$$\lim_{x \to \infty} \left(1 + \frac{3}{x}\right)^{3 x}$$
transform
do replacement
$$u = \frac{x}{3}$$
then
$$\lim_{x \to \infty} \left(1 + \frac{3}{x}\right)^{3 x}$$ =
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{9 u}$$
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{9 u}$$
=
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{9}$$
The limit
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}$$
is second remarkable limit, is equal to e ~ 2.718281828459045
then
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{9} = e^{9}$$
The final answer:
$$\lim_{x \to \infty} \left(1 + \frac{3}{x}\right)^{3 x} = e^{9}$$