#### Answer

All real numbers.

#### Work Step by Step

The domain of $f(x)=3x$ is the set of all real numbers $x$ for which $3x$ is defined as a real number. We know if we multiply two real numbers, we get another real number, so $3x$ is a real number for all real numbers $x$. Therefore, the domain of $f(x)=3x$ is all real numbers.
In set notation, the domain is $\{x: x\in R\}.$
In interval notation, the domain is $(-\infty , \infty).$