I'm assuming the function given is y = 9300(0.991)^x
If so, then the base of the exponent 0.991 is in the form 1+r
1+r = 0.991
r = 0.991-1
r = -0.009
The negative r value indicates a percent decrease.
Specifically it is a 0.9% decrease since 0.009 = 0.9/100 = 0.9%
Any time you have a percent decrease like this, the exponential function is undergoing decay.