Well, whoever gave you this problem expects you to know thedefinitionof "equivalence relation"!

An equivalence relation, xRy, on a set is an equivalence relation if

1) It is "reflexive": xRx for every x in the set.

Suppose f is a function fromRtoR. Is fRf? That is, is f(0)= f(0)?

2) It is "symmetric ": if xRy then yRx.

Suppose f and g are such functions and fRg (that is, f(0)= g(0)). Is gRf (is g(0)= g(0))?

3) It is "transitive": if xRy and yRz then xRz.

Suppose f, g, and h are such functions, fRg (that is, f(0)= g(0)) and gRh (that is, g(0)= h(0)). Is fRh (is f(0)= h(0))?

[f] is the "equivalence class" of f, the set of all functions that are equivalent to f(x)= x. If g(x) is equivalent to that, if xRg, what must be true about g(x)?