## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

The domain is $\{x\in\mathbb{R}|x\leq-2\}$.
For a square root to be defined, the function inside the root must be greater than or equal to $0$. The domain is defined by applying the requirement: $-x-2\geq0$ $-x-2+x\ge0+x$ $-2\geq x$ $x\le-2$ Therefore ,the domain is $\{x\in\mathbb{R}|x\leq-2\}$.