Let's take the limit
$$\lim_{x \to \infty} \left(1 + \frac{1}{7 x}\right)^{5 x}$$
transform
do replacement
x
u = ---
1/7then
$$\lim_{x \to \infty} \left(1 + \frac{1}{7 x}\right)^{5 x}$$ =
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{\frac{5 u}{7}}$$
=
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{\frac{5 u}{7}}$$
=
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{\frac{5}{7}}$$
The limit
$$\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}$$
is second remarkable limit, is equal to e ~ 2.718281828459045
then
$$\left(\left(\lim_{u \to \infty} \left(1 + \frac{1}{u}\right)^{u}\right)\right)^{\frac{5}{7}} = e^{\frac{5}{7}}$$
The final answer:
$$\lim_{x \to \infty} \left(1 + \frac{1}{7 x}\right)^{5 x} = e^{\frac{5}{7}}$$