For this case we have by definition, that the Greatest Common Factor or GFC, of two or more whole numbers is the largest integer that divides them without leaving a residue.
Now, we look for the factors of 72 and 210:
72: 1,2,3,4,6,8,9,12,16,24,36,72
210: 1,2,3, 5,6,7 ...
Thus, the GFC of both is 6.
Then, the GFC of and is given by:
ANswer: