Hola
Es así
\( f'(3)=\displaystyle\lim_{x \to 3}\dfrac{f(x)-f(3)}{x-3}=\displaystyle\lim_{x \to 3}\dfrac{\frac{x}{x-2}-\frac{3}{3-2}}{x-3}=\lim_{x \to 3}\dfrac{\frac{x-3x+6}{x-2}}{x-3}=\lim_{x \to 3}\dfrac{\frac{-2(x-3)}{x-2}}{x-3}=\lim_{x \to 3}-\dfrac{2}{x-2}=-\dfrac{2}{3-2}=-2 \)
Saludos