Each of the domain values, x, should only output a single unique range value, y.
This example is a relation and not a function because the the domain value of -1 output 2 distinct range values.
-1 outputs a 3
and
-1 outputs a 5
If I were to draw a vertical line at the domain value of -1 that vertical line would intersect the points (-1,3) and (-1,5). Which indicates that the relation is not a function.
