1/3(3x-6)-2/7(7x-21)=9 

х-2-2х+6=9

-х=9-6+2

х=-5

1/3*(3х-6) - 2/7*(7х-21) = 9

1) 1/3*3х = 3х/3 = х

2) 1/3*6 = 6/3 = 2

3) 2/7*7х = 2*х = 2х

4) 2/7*21 = 2*3 = 6

х-2-2х+6 = 9

-х = 9-6+2

-х = 5

х = -5

Проверяем:

1/3*(3*(-5)-6)-2/7*(7*(-5)-21)=1/3*(-21)-2/7*(-56) = -7+16 = 9

Ответ: -5.