How can these graphs be plannar? Is there a good way to prove it?

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I don't know how could I prove these graphs are plannar or not, the book does not give examples how to do so and i'm consufed. Can someone show me how to do it? I'd appreciate

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The graph 'a' is not planar since it contains the subgraph

$\{\{7,8\},\{1,8\},\{1,7\},\{2,7\},\{2,8\},\{4,7\},\{4,8\},\{1,2\},\{1,4\},\{2,4\}\}$

which is isomorphic to $K_5$, the complete graph with $5$ vertices.

https://en.m.wikipedia.org/wiki/Planar_graph