You can't differentiate your function? Differentiation is like an onion: you have to peel back the layers, in inverse order in which you would compute. Let's say you take your function
$\displaystyle f(x)=\dfrac{1}{3}(x-3)\sqrt{x},$
and you want to find $\displaystyle f(3).$ List all the computations you must do, in order, in order to find this value.