How to tell if an equation is a function or not

You can tell an equation is a function because every element in the domain has only 1 element in the range

For instance, lets take this set. (2,3)(4,5)(6,7)
The domain is 2,4,6 and our range is 3,5,7 for ever range there is only one unique range that goes with it.

Now, lets take this set. (2,4)(2,4)(3,5)
Here our domain is 2,2,5 and our range is  4,4,5
Here we can see that both 2 points to 4 so this is not a function.