(Х-21/31)+14/31= 25/31

( Х - 21/31 ) + 14/31 = 25/31
 Х - 21/31 = 25/31 - 14/31
 Х - 21/31 = 11/31
 Х = 11/31 +21/31
  Х = 32/31 или 1 1/31
 проверяем: ( 32/31 - 21/31 ) + 14/31 = 25/31  всё верно