( -31/21х + 5/7) - 1/3 = - 18/7

-31/21х  +15/21  -7/21  = -54/21

-31/21х=-54/21  -15/21  + 7/21

-31/21х= -62/21

х= -62/21 : -31/21

х=-21/21 * (-21/31)

х=2

Проверка:

((-31/21)*2 + 5/7)-1/3=-18/7

-62/21 + 5/7 - 1/3 =-18/7

-62/21+15/21-7/3=-18/7

-62/21 + 15/21 -7/21=-18/7

-54/21=-18/7

-18/7=-18/7

(-31/21х+5/7)-1/3 = -18/7

-31/21х+5/7 = -18/7+1/3

-31/21х+5/7 = -54/21+7/21

-31/21х+5/7 = -47/21

-31/21х = -47/21-5/7

-31/21х =-47/21-15/21

-31/21х = -62/21

х = -62/21:(-31/21)

х = -62/21*(-21/31)

х =  2

Проверяем:

(-31/21*2+5/7)-1/3 = (-62/21+15/21)-1/3 = -47/21-1/3 = -47/21-7/21 = -54/21 =

= 18/7 

Ответ: 2.