If $e^x + e^{f(x)} = e$, then the domain of $f(x)$ is

If $f(x) = \dfrac{4x + 3}{6x - 4}$, $x \neq \dfrac{2}{3}$ and $(\text{fof})(x) = g(x)$ where $g: \mathbb{R} - \left\{\dfrac{2}{3}\right\} \rightarrow \mathbb{R} - \left\{\dfrac{2}{3}\right\}$, then $(g\,o\,g\,o\,g\,o\,g\,o\,g)\,(3) =$

If $f(x-y) + f(x+y) = 2f(x)f(y)$ for all $x, y \in \mathbb{R}$, then $f(x)$ is .....