X + (x+1) + (x+4) + (х+7) = 28

x + (x + 1) + (x + 4) + (х + 7) = 28;

Раскроем скобки.

x + x + 1 + x + 4 + x + 7 = 28;

Сгруппируем х в скобки.

(x + x + x + x) + (1 + 4 + 7) = 28;

x * (1 + 1 + 1 + 1) + (5 + 7) = 28;

x * 4 + 12 = 28;

4 * x + 12 = 28;

4 * x = 28 - 12;

4 * x = 16;

x = 16/4;

x = 4;

Ответ: х = 4.