Answer
$\log_b \dfrac{c}{d}$
Work Step by Step
Using $\log_b x-\log_b y=\log_b \dfrac{x}{y}$ (or the logarithm of a quotient), the given logarithmic equation, $
\log_b c-\log_b d
,$ is equivalent to
\begin{array}{l}\require{cancel}
\log_b \dfrac{c}{d}
.\end{array}