Решение:

А (12 + 3у) : 21 = 6

12 + 3y = 6 * 21

12 + 3y = 126

3y = 126 - 12

3y = 114

y = 38

Б) 45 - 2(3x - 8) = 31

2(3x - 8) = 45 - 31

6x - 16 = 14

6x = 14 + 16

6x = 30

x = 5

в) 6y - y - 35 = 50

5y = 50 + 35

5y = 85

y = 17