The general exponential model is given in the form:

where y is the value at time t, a is the initial value, k is the growth/decay rate, and t is the time.
Note that if k is greater than 0, it represents a growth function, while if k is less than 0, the function represents a decay model.
From the question, the function is given to be:

This means that we have the following parameters:

Therefore, the decay rate is -2.5.