Consider the following graph.

Consider the graph below: The objective is to check whether the curve is the graph of a function of X or not. The vertical line test: Consider a graph. Draw a vertical line anywhere on the graph. If no vertical line cuts the graph at more than one point, then the graph is a function. Draw the vertical lines on the graph above. Since no vertical line cuts the graph at more than one point, therefore, the curve is the graph of a function of 2c