acc to handshaking lemma ,
sum of all degree = 2 *|E|
given , 3-regular graph
3 V = 2|E|
V = 2 E / 3
every planar region is bounded by exactly 5 edges
assume , there is F --- > faces
for a planar graph with vertices
sum of degree (faces) = 2*E
5 F = 2 E
F = 2E / 5
now put the V and F in
V -E + F = 2
and got E = 30 edges