In a function, each x value can only have one y value. To test this, you can try the vertical line test. Put a vertical line on the function; if it touches only one point the graph is a function. For graph 4, the x value of 3 has y values of 1 and 4 so it is not a function. For graph 5, the x values of 1 and 5 have many y values so the graph isn't a function. For graph 6, each x value has only one y value so it is a function.