Answer
Yes.
$$y=- \sqrt{25-x^2}$$
Work Step by Step
Yes, it is a function since there exists no real number for which there are two distinct values of $y$.
This graph is the negative section of the circle$$x^2+y^2=25 \quad \Rightarrow \quad y= \pm \sqrt{25-x^2},$$so the function representing this section is $$y=-\sqrt{25-x^2}.$$
