Verified answer

x/4 + 5x/7 - x/28 = 4

приводим к общему знаменателю 28

7*x/28 + 5*4x/28 - x/28 = 4

7x/28 + 20x/28 - x/28 = 4

26x/28 = 4

13x/14 = 4

x = 14*4/13 = 56/13 = 4 4/13