Can somebody walk me through this answer

I have these:

$\displaystyle w = ∂F(K, AL) / ∂L$ and $\displaystyle r = ∂F(K, AL) / ∂K$.

Where $\displaystyle K = K/AL$

I have the answer $\displaystyle A[ f (k) - kf '(k)]$ but confused on the steps to prove it

Thanks