b. All real numbers :)
The only conern here is that you are able to square x. Since the square of any real number gives a real number answer, then that is the domain.
If the function was [tex]\sqrt{x}[/tex], then the domain woudl be all POSITIVE real numbers since you cannot take the square root of a negative number. Try it on your calculator! you will get an error! Domain simply means all possible values for x that actually give real number answers.
(the opposite of a real number is an imaginary number, like 'i' = [tex]\sqrt{-1}[tex]. You can see that you cannot sqrt a negative number, yet, in quantum mechanics 'i' is used all the time because it makes things work in calculations.
