PD= 600(0.978)^t

PD is decreasing at 2.2% per year.

How did they get 2.2%

.978 = 1 - .022 , since it represents a decrease

= 100% - 2.2%

the actual "formula" could be written as
PD = 600(1 - .022)^t

Had there been an increase of 2.2% you would have
600(1 + .022)^t