Answer
$\{ln3, ln5 \}$
Work Step by Step
Step 1. Let $u=e^x$, we have $u^2-8u+15=0$ or $(u-3)(u-5)=0$
Step 2. For $u=3$, we have $e^x=3$, thus $x=ln3$
Step 3. For $u=5$, we have $e^x=5$, thus $x=ln5$
Step 4. Write the answers in set form $\{ln3, ln5 \}$