Answer:
They must convince 5 members.
Explanation:
Given,
The total number of council members = 24,
Since, 2/6 of the council members say they already support the idea,
So, the number of members who already support the idea =

= 8
Now, the number of members who have to support = 13,
Hence, the remaining members who must support the idea = total members who have to support - members who already support
= 13 - 8
= 5