Answer
$\frac{ln{3}}{2}$.
Work Step by Step
If $e^{-2x}=\frac{1}{3}$, then by taking the natural logarithm of both sides we get $-2x=ln{\frac{1}{3}}$, therefore $x=\frac{ln{\frac{1}{3}}}{-2}=\frac{-ln{3}}{-2}=\frac{ln{3}}{2}$.
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