Since the function is y = 3x, take any value of x and multiply it to get y.
If the y-coordinate is not 3 times the x-coordinate, it is not on the graph.
0 * 3 = 0, so (0, 0) is on the graph.
5 * 3 = 15, so (5, 15) is on the graph.
9 * 3 = 27, not 3, so (9, 3) is not on the graph.
7 * 3 = 21, so (7, 21) is on the graph.
