Changing the Base

By definition, $x = b^{\log_b(x)}$. Taking the log base $a$ of both sides gives

$$\log_a(x) = \log_b(x) \log_a(b)$$

which tells how to convert the base from $b$ to $a$, that is, how to convert the log base $b$ of $x$ to the log base $a$ of $x$. (Just multiply by the log base $a$ of $b$.)