Bonsoir,
(4) - y + 9 = 8
- y = 8 - 9
- y = - 1
y = 1
(5) 4y - 5 = 12
4y = 12 + 5
4y = 17
y = 17/4
(6) 3y - 11 = 0
3y = 11
y = 11/3.