#### Answer

... $5-x>0$

#### Work Step by Step

Book reference:
Last sentence before example 7.
The domain of $f(x)=\log_{b}g(x)$ consists of all $x$ for which $g(x)>0$.
In other words, the logarithmic function is defined only for positive arguments,
so $\log_{5}(5-x)$ is defined only if
$5-x>0$