5 3/7 - (b - 2 1/7) = 2 1/4
5 3/7 - b + 2 1/7 = 2 1/4
7 4/7 - b = 2 1/4
b= 7 4/7 - 2 1/4 = 7 16/28 - 2 7/28
b= 5 9/28
проверим:
5 3/7 - ( 5 9 /28 - 2 1/7 ) = 2 1/4
5 3/7 - ( 5 9/28 - 2 4/28) = 2 1/4
5 3/7 - 3 5/28 = 2 1/4
5 12/28 - 3 5/28 = 2 1/4
2 7/28 = 2 1/4
2 1/4 = 2 1/4