Using Euler’s formula find the unknown

By Euler's formula, we have

F + V - E = 2

(i) F + 6 - 12 = 2

F - 6 = 2

F = 8

(ii) 5 + V - 9 = 2

V - 4 = 2

V = 6

(iii) 20 + 12 - E = 2

32 - E = 2

E = 30

Thus, the table can be completed as: