# What is the domain of f(x) = x?

All real values of $x$.
The "domain" of a function is the set of values that you can put into the function such that the function is defined. It's easiest to understand this in terms of a counter-example. For instance, $x = 0$ is NOT part of the domain of $y = \frac{1}{x}$, because when you put that value into the function, the function isn't defined (i.e. $\frac{1}{0}$ isn't defined).
For the function $f \left(x\right) = x$, you can put any real value of $x$ into $f \left(x\right)$ and it will be defined - so that means the domain of this function is all real values of $x$.