how to find the value of $\log_3 7$ Can I ask how to compute $\log_3 7$, using the changing the base of logarithm.

If you mean, "How can I calculate $\log_3 7$ using the change of base formula?":
I've never memorized the change of base formula, I always re-derive it as needed. The key is to remember what the expression means: $\log_3 7 = r$ means that $3^r = 7$. Taking logarithms base $b$ on both sides, we have
$$\begin{align*}
3^r &= 7\\
\log_b(3^r) &= \log_b(7)\\
r\log_b 3&= \log_b 7\\
r &= \frac{\log_b 7}{\log_b 3}\\
\log_3 7 &= \frac{\log_b 7}{\log_b 3}.
\end{align*}$$
So if you want to compute $\log_3 7$ using the natural log, you would have
$$\log_3 7 = \frac{\ln 7}{\ln 3}.$$
If you want to compute them using the common logarithm (base 10), you would compute
$$\log_3 7 = \frac{\log 7}{\log 3}.$$
