The given function is $y = 5x - 9$.

This is a linear function of the form $y = mx + b$, where $m = 5$ and $b = -9$. Since linear functions are defined for all real numbers, the domain of this function is the set of all real numbers.

Domain: $\mathbb{R}$ (or $(-\infty, \infty)$)

Would you like more details on why the domain includes all real numbers?

Here are 5 related questions for practice:

1. What is the range of the function $y = 5x - 9$?
2. How do you find the x-intercept of $y = 5x - 9$?
3. How do you find the slope of the line represented by $y = 5x - 9$?
4. If $y = 5x - 9$, what is the value of $y$ when $x = 3$?
5. What is the graph of the function $y = 5x - 9$ like?

Tip: Linear functions always have a domain of all real numbers because there's no restriction on the values that $x$ can take.