A simple graph has 9 edges, 3 vertices of degree 4 and all others of degree 2. How many vertices does the graph have?

A simple graph has 9 edges, 3 vertices of degree 4 and all others of degree 2. How many vertices does the graph have?

FORMULA

TOTAL OF VERTICES × DEGREE= 2 × EDGES 

Let total vertices= x

3×4+(x-3)2= 2×9

12+2x-6= 18

x=(18-12+6)/2

ANSWER IS 6

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