Here is the question I am trying to solve:

I imagine that I would begin by trying to prove that:Let R be a relation from A to B and S be a relation from B to C

Prove that: Dom($\displaystyle S \circ R$) $\displaystyle \subseteq $ Dom(R)

$\displaystyle \varphi \in$ Dom ($\displaystyle S \circ R$) $\displaystyle \Rightarrow \varphi \in$ Dom(R)

however, I have no idea how to go about doing this. no doubt this stems from my inability to define the domain of a composite function

any help would be much appreciated!