Chapter 4 - An Introduction to Functions - 4-5 Writing a Function Rule - Practice and Problem-Solving Exercises - Page 271: 13

It is not a function

A vertical line at x=1 intersects two points on the graph. In order to pass the vertical line test, we must be able to put a vertical line anywhere on the graph and not have it intersect the curve on the graph multiple times. Thus, this is not a function as it fails the vertical line test.