1. The data is not linear. There's not a constant difference between the f(x) values. (AKA there's not a fixed slope between the data points)
2. The data is exponential, because there's a consistent ratio in the f(x) values:
189/567 = 1/3
63/189 = 1/3
21/63 = 1/3
7/21 = 1/3
Each new y-value (aka f(x)-value) is 1/3 the previous value.
Also, f(0)=567.
If we put that together, we have f(x) = 567 (1/3)^x.
