Draw two ovals labeled "input" and "output". The terms "domain" and "range" will work as well.
The values -3, -1, 1, 3 go in the "input" oval. These are the x values.
The y values 2, 0, 2, 4 go in the "output" oval. Let's remove the second copy of "2" and we get the set 2, 0, 4
Check out the diagram below.
If you were to plot all of the points on the xy grid, then none of the points vertically align. It means we cannot draw a single vertical line through 2 or more points. Therefore, the data points pass the vertical line test. This relation is a function.