For a real number $t$, let $f(t)$ denote the slope of the line passing through the origin and tangent to the curve $y = \frac{1}{e^x} + e^t$. For the constant $a$ satisfying $f(a) = -e\sqrt{e}$, find the value of $f'(a)$. [3 points]\\
(1) $-\frac{1}{3}e\sqrt{e}$\\
(2) $-\frac{1}{2}e\sqrt{e}$\\
(3) $-\frac{2}{3}e\sqrt{e}$\\
(4) $-\frac{5}{6}e\sqrt{e}$\\
(5) $-e\sqrt{e}$