#### Answer

Domain: $(-\infty,\ \infty)$
Range: $(-\infty,\ \infty)$

#### Work Step by Step

We are given:
$f(x)=3x$
The domain consists of all values that $x$ is allowed to be. In this case, there are no restrictions on $x$, so the domain is all real numbers. Domain: $(-\infty,\ \infty)$.
The range consists of all values that $y$ can be. Since this is a graph of a diagonal line, $y$ takes on all real values. Range: $(-\infty,\ \infty)$.