How to differentiate without knowing the function, only the inputs?

Question

Given a function $f(p, \alpha w)$, where $\alpha > 0$, how do I differentiate with respect to $\alpha$ ? The answer is supposedly $ w D_w f(p,w) $ (it says that it differentiated with respect to $\alpha$ and evaluated at $\alpha =1$). I don't fully understand why this is the case.