First of all, read the question once again, very carefully, look especially on the f(x) = x part, because I think you misunderstood something a bit. But I also see you sort of found the clue. Think about synonyms to the words you gave.

Still not exactly. It is said that for the four given values of x, it is true that f(x) = x. In other words

Spoiler (click to show/hide)

f(height above ground) = height above ground

and so on. And think about some synonyms in the fifth equation.