Given,
$V=6$
Each vertex has degree $4$.
So, sum of degrees is
$6 \times 4=24$
By handshaking theorem,
$2E=24$
$E=12$
For a connected planar graph, Euler’s formula is
$V-E+R=2$
Substitute $V=6$ and $E=12$.
$6-12+R=2$
$R=8$
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