The function f(t) = 15,000(0.86)t models the value in dollars of a used car t years after it is purchased. is the function an exponential growth function or an exponential decay function?

An exponential function is given by;
f(x)=ba^x
where a is any value greater than 0, 
           b is the intial value
when a>1 then f(x) is defined as an exponential growth
when a<1 then f(x) is defined as an exponential decay
From the our function;
f(t)=15000(0.86)^t
a=0.86<1
b=15000
since a<1, the function will be an exponential decay.