Detail solution
Let's take the limit
$$\lim_{x \to \infty} \left(\frac{3 x - 4}{3 x + 2}\right)^{2 x}$$
transform
$$\lim_{x \to \infty} \left(\frac{3 x - 4}{3 x + 2}\right)^{2 x}$$
=
$$\lim_{x \to \infty} \left(\frac{\left(3 x + 2\right) - 6}{3 x + 2}\right)^{2 x}$$
=
$$\lim_{x \to \infty} \left(- \frac{6}{3 x + 2} + \frac{3 x + 2}{3 x + 2}\right)^{2 x}$$
=
$$\lim_{x \to \infty} \left(1 - \frac{6}{3 x + 2}\right)^{2 x}$$
=
do replacement
$$u = \frac{3 x + 2}{-6}$$
then
$$\lim_{x \to \infty} \left(1 - \frac{6}{3 x + 2}\right)^{2 x}$$ =
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{- 4 u - \frac{4}{3}}$$
=
$$\lim_{u \to \infty}\left(\frac{\left(1 + \frac{1}{u}\right)^{- 4 u}}{\left(1 + \frac{1}{u}\right)^{\frac{4}{3}}}\right)$$
=
$$\lim_{u \to \infty} \frac{1}{\left(1 + \frac{1}{u}\right)^{\frac{4}{3}}} \lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{- 4 u}$$
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{- 4 u}$$
=
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{-4}$$
The limit
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}$$
is second remarkable limit, is equal to e ~ 2.718281828459045
then
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{-4} = e^{-4}$$
The final answer:
$$\lim_{x \to \infty} \left(\frac{3 x - 4}{3 x + 2}\right)^{2 x} = e^{-4}$$