The domain of values for which this function will have a solution is simply all values except the ones that cause division by zero. So for this example if x = -3 or x = 1 there would be division by zero.
So the domain is all real values except x = 1 or x = -3.
I'm reviewing Meirovitch's "Methods of Analytical Dynamics," and I don't understand the commutation of the derivative from r to dr:
$$
\mathbf{F} \cdot d\mathbf{r} = m \ddot{\mathbf{r}} \cdot d\mathbf{r} = m\mathbf{\dot{r}} \cdot d\mathbf{\dot{r}}
$$