The formula to get the multiplicative rate of change is this:
[ f(x2) / f(x1) ] / (x2 - x1)
or
(y2 / y1) / (x2 - x1)
Choosing of the coordinates (-1, 0.8) and (0,2)
MRC = 2/0.8 / (0-(-1)) = 5/2
Using the other coordinates (0,2) and (1,5)
MRC = 5/2 / (1-0) = 5/2
The MRC will be constant per interval.