3:6х +7= 3:5х-7

3/6х-3/5х = -7-7

15/30х-18/30х = -14

-3/30х = -14

-1/10х=-14

10х = 1/14

х =1/14:10

х = 1/14*1/10 

х = 1/140

3:6*1/140+7 = 3:5*1/140 -7

3: 6/140+7 = 3:5/140-7

3*140/6+7  = 3*140/5 -7

140/2 +7 = 420/5-7

70+7 = 84-7 = 77 

Ответ: 1/140.

/ - дробь 

3/6x + 7 = 3/5x -7 | всё умноажем на 30x

15 + 210x = 18 - 210x

420x = 3

x = 3/420 = 1/140

Ответ: x = 1/140