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