Let $\log_a$ be the D866: Standard real logarithm function in base $a \in (0, \infty) \setminus \{ 1 \}$.
Let $b \in (0, \infty) \setminus \{ 1 \}$ be a D5407: Positive real number.

Then \begin{equation} \log_a x = \frac{\log_b x}{\log_b a} \end{equation}