There is a rule for functions:
One input(x-value) can only have one output(y-value).
If one input has more than one output, it is not a function.
(This doesn't apply to outputs, one output can have more than one input and still be a function)
This graph shows a set of ordered pairs that does NOT represent a function because there are two points on x = -3. The input -3 has more than one output of -4 and 4, so it is not a function.
