Answer: Set of all real numbers
As an inequality, the domain is
to represent x being between negative infinity and positive infinity.
That shortens down to the interval notation
The reason why the domain is "all real numbers" is because we don't have any issues like division by zero errors, or we don't have negative values under a square root. Replace x with any real number to get some output for f(x).
Visually the graph is a parabola the stretches infinitely to the left and right to confirm the correct domain.