the answer would be A. because two negatives equal a positive
You need to find the common primes...
18=2*3*3 and 72=2*2*2*3*3 so they have 2*3*3 in common and
2*3*3=18
So 18 is the greatest common factor.
The answer to the question is Robert grew 1 2/3 inch taller that his brother
