If you are insecure whether your relation is a function or not you can draw a vertical line right through your graph. If the relation is not a function the graph contains at least two points with the same x-coordinate but with different y-coordinates.
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The relation portrayed in the graph to the left shows a function whereas the relation in the graph to the right is not a function since the vertical line is crossing the graph in two points.
In order for a line to represent a function it must pass the vertical line test.
To apply the horizontal line test, take a pencil and hold it vertically. Then, move it across the graph from left to right. If it intersects the line at 1 point everywhere along the line, then it is a function (functions can only have 1 Y value for every X value).
