rememeber
[tex] \frac{dy}{dx} (ax)=a (\frac{dy}{dx} x)[/tex] and
[tex] \frac{dy}{dx} x^m=mx^{m-1} [/tex]
so
[tex] \frac{dy}{dx} (\frac{-11}{x} )=-11( \frac{dy}{dx} \frac{1}{x})=-11 (\frac{-11}{x} x^{-1})[/tex]=[tex]-11( -1x^{-1-1})=-11(-x^{-2})=-11( \frac{-1}{x^2})= \frac{11}{x^2} [/tex]
at x=9
11/9^2=11/81
f'(9)=11/81
