Chapter P - P.3 - Functions and Their Graphs - Exercises - Page 27: 44

Yes, y is a function of x as for every x- value, there is only 1 y-value

You can determine whether or not a graph is a function of another graph by using a concept called the vertical line test. As you can see, if we draw a vertical black line through the purple parabola, the straight black line intersects the graph twice therefore, it is not a function as for every x-value, there is more than 1 y-value. Thus, y is a function as for every x-value there is only 1 y-value