Answer:
y = 21500 (.91)^t
Explanation:
The equation for exponential growth (or decay) is
y =ab^x
where a is the initial value
and b = 1 - growth (or decay) rate
We know the initial value is 21500
and the decay rate is 9%
b = 1-.09
b = .91
y = 21500 (.91)^t