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.
Answer: a decay function is a*b^(-x)