#### Answer

$y \text{ is a function of }x
\\\text{Domain: }
[0,\infty)$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To determine if the given equation, $
y=-\sqrt{x}
,$ is a function, check if $x$ is unique for every value of $y.$
To find the domain, find the set of all possible values of $x.$
$\bf{\text{Solution Details:}}$
For each value of $x,$ getting the negative square root will produce a single value of $y.$ Hence, $y$ is a function of $x.$
The variable $x$ in the radicand of a radical with an even index cannot be negative. Hence, the domain is the set of nonnegative numbers.
Hence, the given equation has the following characteristics:
\begin{array}{l}\require{cancel}
y \text{ is a function of }x
\\\text{Domain: }
[0,\infty)
.\end{array}