Suppose f(x) = abx is an exponential growth function. which function, then, represents exponential decay?

The growth function given is f(x) = a*(b)^x

=> a is positive and b is greater than 0.

Then, a decay function is a*(b) ^(-x).

To see that you can two functions for a = 1 and b=2.

This table may help you

x             2^x                 2^-x

-10         0.00098            1024

-5           0.03125             32

0            1                        1

5            32                      0.03125

10          1024                  0.00098

As you see 2^x grows exponentially and 2^-x decays exponentially.

a decay function is a*b^(-x)

These are the multiples of 15 but what you are probably asking or what are the factors of 15