{(2, 2),(4, 4),(6, 6),(8, 8)} Is a function.
{(0, 3),(3, 5),(5, 6),(8, 4)} Is a function.
{(1, 2),(3, 3),(4, 8),(6, 3)} Is a function.
{(3, 4),(5, 2),(5, 6),(7, 3)} Is not a function.
For a relation to be a function, every x value must have only one y value.