It is an exponential decay curve.
The general form of these is y = k.a^x where 0 < a < 1 [negative values for a are much more complicated]
Some key points can be read from the graph - (0, 3) (1, 1) and less accurately (-1, 9) and (2, 1/3)
This points the the equation y = 3(1/3)^x