Answer
$(-\infty, +\infty); x \ne \frac{2}{3}$
Work Step by Step
Recall that for a function and its inverse, the domain and the range are switched. The range for the original function is: $(-\infty, +\infty); y \ne \frac{2}{3}$. Thus, the domain for the inverse function is this range, namely: $(-\infty, +\infty); x \ne \frac{2}{3}$