Question on graph theory

Question

Please explain how did they get that equation in E and V.

Comments

But it belongs to planarity which is out of syllabus

Answer 1

connected planar graph with no cycle of length less than 4 means that no circuit of length 3 .so we have theorem in this case:e<=2n-4
where n=15 given put and get the answer.
e<=30-4

e<=26
so 26 edges needed in connected planar graph(CPG)

in all other cases we have to take e<=3n-6

Comments

Can you give me a source where I can read all these theorems?