Solución detallada
Tomamos como el límite
$$\lim_{x \to \infty} \left(\frac{x + 7}{x + 5}\right)^{x + 3}$$
cambiamos
$$\lim_{x \to \infty} \left(\frac{x + 7}{x + 5}\right)^{x + 3}$$
=
$$\lim_{x \to \infty} \left(\frac{\left(x + 5\right) + 2}{x + 5}\right)^{x + 3}$$
=
$$\lim_{x \to \infty} \left(\frac{x + 5}{x + 5} + \frac{2}{x + 5}\right)^{x + 3}$$
=
$$\lim_{x \to \infty} \left(1 + \frac{2}{x + 5}\right)^{x + 3}$$
=
hacemos el cambio
$$u = \frac{x + 5}{2}$$
entonces
$$\lim_{x \to \infty} \left(1 + \frac{2}{x + 5}\right)^{x + 3}$$ =
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{2 u - 2}$$
=
$$\lim_{u \to \infty}\left(\frac{\left(1 + \frac{1}{u}\right)^{2 u}}{\left(1 + \frac{1}{u}\right)^{2}}\right)$$
=
$$\lim_{u \to \infty} \frac{1}{\left(1 + \frac{1}{u}\right)^{2}} \lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{2 u}$$
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{2 u}$$
=
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{2}$$
El límite
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}$$
hay el segundo límite, es igual a e ~ 2.718281828459045
entonces
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{2} = e^{2}$$
Entonces la respuesta definitiva es:
$$\lim_{x \to \infty} \left(\frac{x + 7}{x + 5}\right)^{x + 3} = e^{2}$$