600 - х = 802 : 2
600 - х = 401
х = 600 - 401
х = 199
----------------------------
600 - 199 = 802 : 2
401 = 401
х + 57 = 7146 * 6
х + 57 = 42876
х = 42876 - 57
х = 42819
---------------------------------
42819 + 57 = 7146 * 6
42876 = 42876
789 + х = 6342 - 699
789 + х = 5643
х = 5643 - 789
х = 4854
-----------------------------------
789 + 4854 = 6342 - 699
5643 = 5643