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Graph[{1\[UndirectedEdge]2,2\[UndirectedEdge]3, 3\[UndirectedEdge]1}, GraphLayout -> "PlanarEmbedding"],
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Graph[{1\[UndirectedEdge]2,1\[UndirectedEdge]3,1\[UndirectedEdge]4,2\[UndirectedEdge]3,2\[UndirectedEdge]4,3\[UndirectedEdge]4},GraphLayout->"PlanarEmbedding"]

Graph[{1\[UndirectedEdge]2,1\[UndirectedEdge]3,1\[UndirectedEdge]4,2\[UndirectedEdge]3,2\[UndirectedEdge]4,3\[UndirectedEdge]4}]