Answer
Yes, f(x) = 3 is a linear function. The slope is 0, and the rate of change is 0.
Work Step by Step
Linear functions are written in the form f(x) = ax+b. In this case, a = 0, so there is only a b value (3). Since a = slope = 0 the slope is 0 and the rate of change (which = the slope) is also 0.
