((Х+11)+(Х:6))*2 = 78

((Х + 11) + (Х/6)) * 2 = 78; 

Раскроем скобки. 

((Х + 11) + (Х/6)) = 78/2; 

((Х + 11) + (Х/6)) = 70/2 + 8/2; 

((Х + 11) + (Х/6)) = 35 + 4; 

((Х + 11) + (Х/6)) = 39; 

x + 11 + x/6 = 39; 

x + x/6 = 39 - 11; 

x + x/6 = 28; 

6/6 * x + 1/6 * x = 28; 

7/6 * x = 28; 

1/6 * x = 4; 

x = 4 * 6; 

x = 24; 

Ответ: х = 24.