When a line has a slope of zero it means that it is horizontal. This means that all the x values have the same y values. In this case it means that the y value is always -1.
let x and y be the number of chip and pretzel bags respectively :
4x%2B2y%3 C=20
:
if x=1 then 4%281%29%2B2y%3C=20--->{{2y<=16}}}--->y%3C=8 :
if x=3 then 4%283%29%2B2y%3C=20--->{{2y<=8}}}--->y%3C=4 :
so chips 1 and pretzels 8 :
chips 3 and pretzels 4.
