To determine whether a relation is a function, we can use the vertical line test. If a vertical line touches the relation in more than one point, it is not a function.
Since a vertical line will touch the relation at two points at x=1
The plot shows (1, 1) and (1, 3) are both defined by the relation. It does not pass the "vertical line test", which requires the relation be single-valued everywhere.
translates to 
