208*3 (624
612:6 (102
109*7 (763
123*4 (492
114*6 (684
735:7 (105
321:3 (107
5/12•7/8=35/96;///
6/7•8/9=16/21;///
12/13•2/3=8/13;///
4/9•27/16=3/4
Х•3=42
Х=42:3
Х=14
Ответ:14
Всё легко и просто
5(х+3)+7=3(х+12)
5х+15+7=3х+36
5х-3х=36-15-7
2х=14
х=14:2
х=7
проверка
5(7+3)+7=3(7+12)
5*10+7=3*19
57=57