There is a rule:
One input(x-value) can only have one output(y-value) in order to be a function.
If one input has more than one output, it is not a function.
(This doesn't apply to outputs, meaning that it's still a function when one output has more than one input, because it's the input that can have only one output)
For example, problem #1, the graph is a function because each input has only one output.
