Answer
$2.771$
Work Step by Step
Change of Base Formula: $\hspace{20pt} \log_ a M = \dfrac{\log_b M}{\log_b a}$
$\log_3 21 = \dfrac{\log_{10} 21}{\log_{10} 3}$
Where: $a=3 \hspace{10pt} M = 21 \hspace{10pt} b = 10$
$ \dfrac{\log_{10} 21}{\log_{10} 3} \approx \boxed{2.771}$
