it will have a constant rate of change if ur numbers are equal
f(x) / x
-20/-5 = 4
-8/-2 = 4
8/2 = 4
16/4 = 4
so yes, it has a constant rate of change (ur slope) and it is 4
u can also find ur slope by using the slope formula : (y2 - y1) / (x2 - x1) and 2 sets of points
(4,16),(2,8)
slope = (8 - 16) / (2 - 4) = -8/-2 = 4 <== ur constant rate of change