Chapter 4 - Applications of the Derivative - 4.2 What Derivatives Tell Us - 4.2 Exercises - Page 256: 3

generic answer: $y=c$ one specific answer: $y=2$

Since there's a point where $f'(x)=0$ and there can't be a min or max at the point, it must be a line with slope of zero. $y=c$, with c being a constant, meets these requirements. Any real number, for example 2, can be c.