Simple graph proof question
I have an exercise about graphs which I cannot solve. Could you please help me?
The exercise is as follows:
Thank you very much
A simple graph, also called a strict graph, is an unweighted, undirected graph containing no graph loops or multiple edges. A well-known theorem states that the sum of the degrees of the vertices of a simple graph equals twice the number of edges of the graph.
Prove the following:
There is no simple graph with 12 vertices and 28 edges so that
(i) all vertices have degree 3 or 4
(ii) all vertices have degree 3 or 6