3:x+x=28

3 : x + x = 28
3/x = 28 - х
х = (28 - х)/3
x = 28/3 - x/3
x + x/3 = 28/3
x * (1 + 1/3) = 28/3
x * (3/3 + 1/3) = 28/3
x * (3+1)/3 = 28/3
x * 4/3 = 28/3
x = 28/3 : 4/3
x = 28/3 * 3/4
x = 3/3 * 28/4
x = 1 * 7
x = 7

Ответ: 7