Answer
It is constant, and equals the slope, m.
Work Step by Step
$\displaystyle \frac{f(b)-f(a)}{b-a} $is the expression we use to calculate the slope between any two points on a line.
Its unit is: (units of $y=f(x)$) per (units of $x$)
It is also the expression used to calculate the average rate of change, which has the same units.
Lines (graphs of linear functions) have a constant slope, $m$.
Thus, the average rate of change over an interval $[a,b]$ is constant, and equals the slope, $m$.
