Question

Fi

Answer 1

@Kevin

Answer 2

\[\frac{\mathrm{d}f}{\mathrm{d}x}\bigg|_{x=c}=\lim_{x\to c}\frac{f(x)-f(c)}{x-c}\]so you need to compute
\[\lim_{x\to 6}\frac{f(x)-f(6)}{x-6}=\lim_{x\to6}\frac{-\frac{9}{x}-\left(-\frac{9}{6}\right)}{x-6}\]

Answer 3

yes

Answer 4

what's the question?