Answer
$x∈[0;3)$;
DOMAIN.
Work Step by Step
This is standard interval notion. What is important to note is that the blank dot at $(1,3)$ signifies that $3$ is not included in the function's domain.
Simply said, all $x$'s of a function represent its domain, while all $y$'s represents its range.
In this case, the shaded set of numbers shows the $x$'s of the function, therefore, it is its domain.
