9 9/28 - (4 5/21 - х) = 6 2/7
4 5/21 - х = 9 9/28 - 6 2/7
4 5/21 - х = 9 9/28 - 6 8/28
4 5/21 - х = 3 1/28
х = 4 5/21 - 3 1/28
х = 4 20/84 - 3 3/84
х = 1 17/84
Или
9 9/28 - (4 5/21 - х) = 6 2/7
9 9/28 - 4 5/21 + х = 6 2/7
х = 6 2/7 + 4 5/21 - 9 9/28
х = 6 24/84 + 4 20/84 - 9 27/84
х = 10 44/84 - 9 27/84
х = 1 17/84