calculus - Could you explain the expansion of $(1+frac{dx}{x})^{-2}$?

Could you explain the expansion of $(1+\frac{dx}{x})^{-2}$?

Source: calculus made easy by S. Thompson.

I have looked up the formula for binomial theorem with negative exponents but it is confusing. The expansion stated in the text is:

$$\left[1-\frac{2\,dx}{x}+\frac{2(2+1)}{1\cdot2}\left(\frac{dx}{x}\right)^2 - \text{etc.}\right]$$

Please explain at a high school level.