{(9,5) , (3,6)} is a function
{(2, 4) , (7,8)} is a function
{(5,5) , (6, 4)} is a function
In a relation
(9,5) is refered to as ordered pair
So a relation is a function if two ordered pair does not have the same value
Most importantly the first value which is the domain.
In (9, 5) , 9 is the domain and 5 is the range or co-domain
In other word a relation is a function if the domain is not the same or repeated in other ordered pairs
So {(5,8), (5,9)} and {(5,1) , (5, 2)}, 5 is domain and it is repeated, that is why it is not a function