Answer
$b+a$
Work Step by Step
Recall:
$\log_a (MN) = \log_a M+\log_a N$
Using the rule above gives:
$\ln6 = \ln(3 \cdot 2)= \ln3+\ln2$
With $\ln3 = b \hspace{20pt} \text{and} \hspace{20pt} \ln2=a$, then
$\ln3+\ln2 = b+a$
Thus,
$\ln6 = \boxed{b+a}$
