Answer:
The answer is 6!!
Step-by-step explanation: Factors of 18: 1, 2, 3, 6, 9, 18
Factors of 66: 1, 2, 3, 6, 11, 22, 33, 66
After listing all of the factors, simply find the number that is shown in both of their factors but find the greatest number.
For example 3 is a factor that is the same but isn't the highest factor that is shown in both, it is 6! Hope this helps and Happy Halloween!!! :D