Which explains why the graph is not a function?
It is not a function because the points are not connected
to each other.
It is not a function because the points are not related by
a single equation.
It is not a function because there are two different x-
values for a single y-value.
It is not a function because there are two different y-
values for a single x-value.
5 4 -3 -2 -1
1 2 3 4
5 x

Look at x = 4. Note how we can draw a vertical line through 4 on the x axis and this single vertical line crosses more than one point. This means the graph fails the vertical line test, and therefore it is not a function.

A single input (x = 4) leads to more than one output (y = -2 and y = 4 at the same time). A function is only possible if any given input in the domain leads to exactly one output.