Yes it is, but it is not a function.
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One way is to try the vertical line test on a graph!
If a vertical line intersects the graph at more than one point then it is not a function.
You use the "vertical line test". If anywhere you can draw a vertical line that goes through two points of the graph, the relation is not a function; otherwise, it is a function. This is just another way of saying that in a function for every x value (input) there is AT MOST one y value (output).
A vertical line!