295 + х = 678 - 117
295 + х = 561
х = 561 - 295
х = 266
------------------------------
295 + 266 = 678 - 117
561 = 561
х + 309 = 716 + 120
х + 309 = 836
х = 836 - 309
х = 527
-------------------------------
527 + 309 = 716 + 120
836 = 836
х - 712 = 900 - 112
х - 712 = 788
х = 788 + 712
х = 1500
--------------------------------
1500 - 712 = 900 - 112
788 = 788
Дабавлено по просьбе :
115 - х = 429 - 37
115 - х = 392
х = 115 - 392
х = - 277
--------------------------------
115 - (- 277) = 429 - 37
115 + 277 = 429 - 37
392 = 392