The function doesn't have inverse because it doesn't pass the horizontal line test. This test tells us that a function has an inverse function if and only if there is no any horizontal line that intersects the graph of at more than one point. As you can see, from the graph of f (the red one), if you draw an horizontal line that passes through then this line will touch the graph of f at three points, so the horizontal line test is not satisfied here. If you see the graph of g, this doesn't represent a function because there is at least one vertical line that touches the graph at more than one point, so this relation is not a function.
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