Answer
$\approx2.771$
Work Step by Step
Using the Change-of-Base Formula, we know that $\log_a{b}=\frac{\log_c{b}}{\log_c{a}}$.
Use the Change-of-Base formula and a calculator to obtain:
$\log_3{21}=\frac{\ln{21}}{\ln{3}}=2.771243749\approx2.771$
