(-31/21х + 5/7) - 1/3 = -2 4/7.
-31/21*x + 5/7 = - 2 4/7 + 1/3.
-31/21*x + 5/7 = -18/7 + 1/3.
-31/21*x + 5/7 = -54/21 + 7/21.
-31/21*x + 5/7 = -47/21.
-31/21*x = -47/21 - 5/7.
-31/21*x = -47/21 - 15/21.
-31/21*x = -62/21.
x = -62/21 : (-31/21),
х = -62/21 * (-21/31),
х = 2