Notice that in that region you have points like (1,1), (2,2), etc.
That means that, for example, if there are 2 pounds of pecans and 2 pounds of walnuts this happens:
If a) is true that means that 2 pounds of pecans - 2 pounds of walnuts = 8 pounds
What happens on the point (6,2)?
We have two points: (0,8) and (8,0)
Obtaining the linear equation:
y=x