Suppose that g(x)=f^{-1}(x). If g(-15)=0, g(0)=3, g(3)=9 and g(9)=20, what is f(f(9))?

The inverse contains the points

 (-15.0), ( 0, 3), (3,9), (9,20)

So the original function contains the points

(0, -15), (3,0), (9,3), (20, 9)

So   f(9)  =   3

And    f(f(9)  =   f(3)      =    0