The answer by Ran G. give a fairly extensive listing of the equivalent models that can be used to specify regular languages (and the list goes on, two-way automata, MSO logic, but that is covered by the link under 'more equivalent models'). And as Raphael stresses, we need an argument to convince the audience that the chosen representation is indeed correct.
Reconsidering the question, it adds 'For instance $\dots$'. That means we have to give a valid construction that, given any of the above models we assume specify language $L$, turns that model into one for $L'$. This generally will be the same type of model, but need not be: we can e.g. start with an deterministic FSA for $L$ and end with a nondeterminitic one for $L'$.
This includes the possibility to use closure operations: in the explicitly given operation in the example we have $L'= (\Sigma^* \setminus L) \cdot \Sigma^*$.
So, my point is that the answer is great, but we should add the "from $L$ to $L'$ construction", when not building a specific language from scratch.