Answer
$(-\infty,3)$
Work Step by Step
Logarithmic functions are defined only for positive arguments.
So, the domain of f is defined with
$3-x \gt 0\qquad $... solve (isolate x)
... add x to both sides
$ 3 \gt x\qquad $ ... or,$\qquad x \lt 3$
The domain of $ f $ is $(-\infty,3)$
