Respuesta :

2/7 - 5/21 = 6/21 - 5/21 (find common denominator first, then subtract)
6/21 - 5/21 = 1/21

3/4 = 63/84
5/7 = 60/84
7/12 = 49/84

63/84 + 60/84+ 49/84 = 172/84
172/84 - 4/84 = 168/84

The answer is 168/84, or 2. Hope this helped!