Answer:
y = 1.8
Explanation:
Question 39).
Let the operation which defines the relation between a and b is O.
Relation between a and b has been given as,
a O b =

Following the same operation, relation between 3 and y will be,
3 O y =

Since 3 O y = 4,

3 + y = 12 - 4y
3 + y + 4y = 12 - 4y + 4y
3 + 5y = 12
3 + 5y - 3 = 12 - 3
5y = 9

y = 1.8
Therefore, y = 1.8 will be the answer.