If you are wondering why E = $\frac{3F}{2}$
In the question it is given “the number of edges on each face is 3”, it means that if we take any face it will be surrounded by 3 edges, now lets say there are 10 faces, so according to question 10*3 = 30 edges must be there.
One more thing is that one single edge divides the faces in two parts (Draw any simple figure you’ll understand), meaning one edge is shared by 2 faces. Acc to our hypothetical example $\frac{30}{2}$=15 edges.
Coming back to the actual question let number of faces be F so we have 3F edges, and since each edge is shared by two faces we have 3F/2, hence E = $\frac{3F}{2}$