13/21 - (х - 2 5/7) = 3/14
х - 2 5/7 = 13/21 - 3/14
х - 2 5/7 = 26/42 - 9/42
х - 2 5/7 = 17/42
х = 17/42 + 2 5/7
х = 17/42 + 2 30/42
х = 2 47/42
х = 3 5/42
13/21 - (х - 2 5/7) = 3/14
х - 2 5/7 = 13/21 - 3/14
х - 2 5/7 = 26/42 - 9/42
х - 2 5/7 = 17/42
х = 17/42 + 2 5/7
х = 17/42 + 2 30/42
х = 2 47/42
х = 3 5/42