May be @Psunny wants to ask the question this way...
If a graph G has 15 edges and G' (G-bar) has 13 edges. How many Vertices does G has .?
To answer this question we know that G U G' is Complete Graph Kn
Therefore G+G' = 15+13 = 28
Total number of edges in the complete graph is nC2
nC2 = (n(n-1))/2 = 28
By solving the equation we get n= 8 or -7
therefore the number of vertices, n = 8