(f o g) (x) means put the whole function, g(x), into x of f(x).
Let's do it:

The graph of this function looks like:
The domain is the set of x values for which the graph of the function is defined for.
Here, you see x = -3 is not included, otherwise the graph is defined for all other values
So the domain would be:
Domain = { x not equal 0 }
