#### Answer

The given relation defines $y$ as a function of $x$.
domain: $(-\infty, +\infty)$
range: $(-\infty, 0]$

#### Work Step by Step

RECALL:
A function is a relation where every value of $x$ is paired with only one $y$ value.
The give equation will give only one value of $y$ for every value of $x$. This means that each $x$ is paired with only one value of $y$.
Thus, the given relation defines $y$ as a function of $x$.
Negative numbers have no real number square roots.
Thus, the value of $x$ cannot be negative.
This means that the domain of the given function is the set of all non-negative real numbers. In interval notation, the domain is $(-\infty, +\infty)$.
The negative square of a number is always 0 or lower.
Thus, the value of $y$ is always less than or equal to zero.
In interval notation, the range is $(-\infty, 0]$.