Model Theory - definiable classes

Hi,

This time I've problem with following exercise:

Let be a first-order language and for each let be a class of -structures.

Show that .

Here are some basic definitions from the book "Model Theory", W. Hodges (which could help):

A **sentence **is a formula with no free variables. A **theory **is a set of sentences.

We say that is a **model **of , or that is **true **in , when holds. Given a theory in , we say that is a **model **of , in symbols , if is a model of every sentence in .

Let be a language and a class of -structures. We define the -**theory **of , , to be the set (or class) of all sentences of such that for every structure in . We omit the subscript when is *first-order*: the theory of , , is the set of all *first-order* sentences which are true in every structure in .

Thanks for any help.