Hello! I don't really understand how to answer the second part of the question (the "hence" part). Please help! Thank you!

f(x) = 2 - (1/4)x^3 , x is a real number, x>0

Sketch the graph of f and find its range.Hencestate the set of values of x for which ff^-1(x) = f^-1f(x)

[By f^-1(x) I mean f inverse. I am not sure of how to type out f inverse on the computer... ]

Thank You!