A good place to start would be consulting the definition of invective and surjective.

A function f : X -> Y is injective (one-to-one) if for any a,b in X, if f(a) = f(b), then a = b. This is a more formal way of saying that distinct elements in X map to distinct elements in Y.

A function f : X -> Y is surjective (onto) if for every element y in Y, there is some element x in X such that f(x) = y. i.e. for every element in the codomain/image/range there is some element in the domain that that maps to it—so no elements in the codomain are “unused” persay.

Define a function between the sets and show that it satisfies these two properties.