[กิตติ]

$\frac{1}{a}=\log_721 \rightarrow \log_73=\frac{1}{a}-1$
$\log_{33}7=M$
$\frac{1}{M}=\log_733 = \log_73+ \log_711$
$\frac{1}{M}=\frac{1}{a}-1+b$
$\frac{1}{M}=\frac{ab-a+1}{a}$
$M=\log_{33}7=\frac{a}{ab-a+1}$