Answer: 11 vertices
Explanation:
Hi, to answer this question we simply have to substitute E=18 and F= 9 on Euler's formula:
F+V = E+2
Where:
F = number of faces
V = number of vertices
E= number of edges
Replacing with the values given:
9+V = 18+2
Solving for V:
V =18+2-9
V = 11
Feel free to ask for more if needed or if you did not understand something.