Proof injective function

[tedjj]

For nonempty sets A,B, and C, let f:A->B and g:B->C be functions. Prove if g o f is one-to-one then f is one-to-one using as many of the following proof techniques as possible: direct proof, proof by contrapositive, and proof by contradiction.
I don't know how to prove it. Can anyone help?

[OfficeShredder]

Do you know what the definition of one-one is? Start with the proof by contrapositive, it's literally two lines.... in case you aren't sure, the contrapositive of your statement is "If f is not one-one, then g o f is not one-one". If the contrapositive of a statement is true, the statement is true

[HallsofIvy]

Continuing with what Office Shredder said, if f is NOT one-to-one, then what must be true? If that's true, what can you say about gof?