Is this graph a function? (hint: use the Vertical Line Test)

When looking at a graph, the vertical line test can help you determine whether or not the curve on the graph is a function. A curve in the coordinate plane is the graph of a function if and only if no vertical line intersects the curve more than once. Note that a vertical line at around x=3.8 (shown below in the graph as the green vertical line) intersects the curve three times. Therefore, this curve is NOT a function.