Answer :

AL2006

To find the GCF:

-- List the factors of the first number.
-- List the factors of the second number.
-- Make a short list of the factors that show up for BOTH numbers.
-- Find the biggest number on the short list.

Factors of the first number (99):  1, 3, 9, 11, 33, 99 .

Factors of the second number (121):  1, 11, 121 .

Short list (factors that show up for both numbers):  1, 11

Biggest number on the short list:  11