We are given the following function which models the decline of a population.

The standard form of such a population decay function is given by

Where a is the initial population, b is the decay rate and x is the time period.
The decay rate is greater than 0 and less than 1
Comparing the given function with the standard form, we see that the decay rate is 0.9
In percentage, the decay rate is 90%

Therefore, the decay rate of the function is 90%