Answer
$\{ln2, ln4 \}$
Work Step by Step
Step 1. Let $u=e^x$, we have $u^2-6u+8=0$ or $(u-2)(u-4)=0$
Step 2. For $u=2$, we have $e^x=2$, thus $x=ln2$
Step 3. For $u=4$, we have $e^x=4$, thus $x=ln4$
Step 4. Write the answers in set form $\{ln2, ln4 \}$