Answer
$$f'(a)=-2a^{-3}$$
Work Step by Step
$$f(x)=x^{-2}$$
That means $$f(a)=a^{-2}$$
The derivative of $f(x)$ at number $a$ can be found as follows: $$f'(a)=\lim\limits_{x\to a}\frac{f(x)-f(a)}{x-a}$$
$$f'(a)=\lim\limits_{x\to a}\frac{x^{-2}-a^{-2}}{x-a}$$
$$f'(a)=\lim\limits_{x\to a}\frac{\frac{1}{x^2}-\frac{1}{a^2}}{x-a}$$
$$f'(a)=\lim\limits_{x\to a}\frac{\frac{a^2-x^2}{a^2x^2}}{x-a}$$
$$f'(a)=\lim\limits_{x\to a}\frac{-(x^2-a^2)}{a^2x^2(x-a)}$$
$$f'(a)=-\lim\limits_{x\to a}\frac{(x-a)(x+a)}{a^2x^2(x-a)}$$
$$f'(a)=-\lim\limits_{x\to a}\frac{x+a}{a^2x^2}$$
$$f'(a)=-\frac{a+a}{a^2a^2}$$
$$f'(a)=-\frac{2a}{a^4}=-\frac{2}{a^3}=-2a^{-3}$$
