factor the numbers, starting from the smallest:
792=2*3*3*11*4
990=2*3*3*11*5
the greatest whole number that can be divided by both 792 and 990 is 2*3*3*11=198
the smallest whole number that can divide both 792 and 990 exactly is 2*3*3*11*4*5=3960
Not sure which one you are asked to find.