# How do you find the domain of f(x) = 9/x?

Domain$\equiv \left(- \infty , 0\right) \cup \left(0 , + \infty\right)$
You have a fractional function. That means that the only problem for the domain is a denominator ($x$) equal to 0, that means $x \ne 0$. So you can take all real numbers but 0.