Answer
$$e^{x}-\frac{1}{3}e^{3x}+c.$$
Work Step by Step
Since $(e^{3x})'=3e^{3x}$, then we have
$$
\int\frac{e^{2x}-e^{4x}}{e^{x}} d x=\int e^{x}-e^{3x}d x \\
=e^{x}-\frac{1}{3}e^{3x}+c.
$$
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