Im not good at math so i use math-way.com
Answer:
(b)
Step-by-step explanation:
Given

Required
The graph
First, make y the subject

Divide through by 3

Let x = 3

Let x = 6

So, we plot the graph through
and 
See attachment for graph
You did the top right but the denominator is 0 so the slope in undefined or 0.
Hope this helps!
Answer:
They will expect the ratio below:
x/20 = 160/8, with x: population
Then
x = 20x160/8 = 400