set up the relation as a table of ordered pairs. Then, test to see if each element in the domain is matched with exactly one element in the range. If so, you have a function!
What makes a function a function is that it only has one input for every output. So in other words the x can't have two y values. But on a graph you can just do the vertical line test to see if the line intersects the vertical line more than once for any x value, it fails. So if we use the vertical line test we can see that for the x value of 2 is intersected by the line more than once, so this isn't a function.
