Hey there!
To find the Greatest Common Factor of the two numbers, 27 and 18, first break each number down into their prime factors:
27= 3*3*3
18=3*3*2
Now, if you look at the prime factors, notice that both numbers have in common the factor 3. Take the greatest amount of times the three is multiplied, 3*3, which is twice, and simplify it.
Therefore, your GCF of the two numbers would be 9.
Hope this helps and have a nice day! :)