Use a regra da cadeia e depois a regra do quociente
[tex]f'(x)=\left[\left(\dfrac{x}{(x-3)^2}\right)^2\right]'\\\\f'(x)=\overbrace{2\left(\dfrac{x}{(x-3)^2}\right)^{2-1}\cdot\underbrace{\left(\dfrac{x}{(x-3)}\right)'}_{\mbox{regra do quociente}}}^{\mbox{regra da cadeia}}\\\\\\f'(x)=2\left(\dfrac{x}{(x-3)^2}\right)\cdot\left(\dfrac{x'(x-3)^2-2(x-3)^{2-1}\cdot x}{(x-3)^4}\right)\\\\\\\boxed{f'(x)=2\left(\dfrac{x}{(x-3)^2}\right)\cdot\left(\dfrac{(x-3)^2-2x(x-3)}{(x-3)^4}\right)}[/tex]
Da pra simplificar algumas coisas, mas isso basta
espero ter ajudado :)
