## College Algebra (6th Edition)

... $5-x>0$
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 $5-x>0$