## Precalculus (6th Edition) Blitzer

In the given graph by the vertical line test, $y$ is not a function of $x$.
According to the vertical line test, if there is any intersection by any vertical line on a graph at more than one point (that is, for one value of x, there is more than 1 value of y), then the graph cannot be defined as y as a function of x. In the graph shown above, for one value of x, there is more than 1 value of y. Thus the graph does not pass the vertical line test. Therefore, $y$ is not a function of $x$. Hence, $y$ is not a function of $x$.