Chapter 2, Functions - Section 2.5 - Linear Functions and Models - 2.5 Exercises - Page 231: 6

Slope and rate of change are zero.

We know that linear functions have the form $y=mx+b$ where $m$=slope, $b$=y-intercept. In this case, we have $y=3$, so the slope is 0 and the y-intercept is 3. Thus the rate of change is 0.