Keep in mind that the original exponential formula was y = abx. You'll see that the b value (growth factor) has been replaced in these new growth and decay functions by either (1 + r) or by (1 - r). The "rate" of growth (r) is calculated as b = 1 + r.
If you know (https://socratic.org/questions/how-do-you-find-the-exponential-model-y-ae-bx-that-fits-the-two-points-0-8-1-3 ) the exponential function find the regression co-efficiant at different points and the use best fit software to find out R.