Answer
$(-\infty, +\infty)$
Work Step by Step
Simplify the right side to obtain:
$f(x) = 3(x) - 3(4)
\\f(x) =3x - 12$
This function is defined for all real number values of $x$, therefore its domain is the set of all real numbers.
Thus, the domain is $(-\infty, +\infty)$.
