Functions cannot have two y-values (outputs) for any single x-value (input), so if you can draw a vertical line that touches more than 1 point on the graph, it is not a function.
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The "vertical line test" will tell you if it is a function or not. The graph is not a function if it is possible to draw a vertical line through two points.
If a vertical line, within the domain of the function, intersects the graph in more than one points, it is not a function.
You use the vertical line test. If you can draw a vertical line though the graph and it intersects it only once, it is a function. If the line crosses the graphs more than once it is not.
Draw a vertical line if the line hits more than one point on the graph then it is not a function.
A graph is represents a function if for every value x, there is at most one value of y = f(x).