Justify why a maximal-planar graph cannot have vertices of degree ≤ 2.
Some definitions: A planar graph is a graph that can be embedded in the plane, it can be drawn on the plane in such a way that its edges intersect only at their endpoints. In other words, it can be drawn in such a way that no edges cross each other.
A planar graph is called maximal planar if adding an edge between any two non-adjacent vertices results in a non-planar graph.
Another exam question that I came across while learning for my upcoming discrete math exam. Unfortunately I have no idea how to solve this problem. Thanks in advance!