Answer:
f(t) = 5×0.87^t
Explanation:
The general form for an exponential function described in this fashion is ...
... f(t) = (starting value) × (1 + (percent change))^t
Here, the "percent change" is -13%, or -0.13.
Then the value (1 + percent change) is (1 + (-0.13)) = 0.87. Putting this and the starting value into the form above, we have ...
... f(t) = 5 × 0.87^t