Can anyone explain to me how can i tell if a relation is a function.
an example can be a parabola or anything related to my question.

A function is a relation such that for each first element ( x- value) there exists one and only one second element.

Another way to say this is that none of the ordered pairs have a repetitive x-value.

example: C= {(2,5) , (6,1) , (8,2), (9,5) , (4,10)}

Set C is a function because each first element is only listed once.