How to prove the existence of bijectivity?

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Cordial greeting.

I recently came across an exercise that asks me to demonstrate bijectivity between two lines in a projective plane.

But firstly, I would like to know how bijectivity is demonstrated to apply it to that topic.

I would be very grateful for any contribution or guide that you give me, thank you from beforehand.

my question is given based on this exercise:

In a projective plane: Given two different lines $L$ and $M$, there is a bijection $f: L \to M.$