Here you have shared two separate functions, f(x) and g(x). You have not indicated how f and g interact. Is g supposed to be the input to f? or what?
The domains of f and g are "the set of all real numbers."
If you meant "find the domain of f(g(x))," then we're finding the domain of
3(x+1)+6, which happens to be the same as before: "the set of all real numbers."
Kindly check to ensure that you have shared ALL of the given problem.