Answer
$3a$
Work Step by Step
With $8=3^3$, then
$\ln8 = \ln(2^3)$
Recall that $\ln \left(M^r\right) = r \ln M$
Using the rule above gives:
$\ln{\left(2^3\right)} = 3 \ln2$
With $\ln2=a$, then
$3 \ln2 = 3a$
Therefore,
$\ln8 = \boxed{3a}$
