For a better understanding of the explanation given here please go through the two graphs provided in the two files attached.
In the first graph we have plotted all the given points from the question. The y-axis is the quantity and the x-axis is the price. As can be seen, as the price increases, the quantity decreases.
In the second graph, the line that connects all the given points is drawn. This line represents the "Demand" Line and is evaluated as follows:
We take any two points (in our case we took the first and the last points, that is, [tex](x_1,y_1)=(0,19)[/tex] and [tex](x_2,y_2)=(3,1)[/tex]) and we made use of the "point slope form" as:
[tex]\frac{y-y_1}{x-x_1}= \frac{y_2-y_1}{x_2-x_1}[/tex]
Thus, we got:
[tex]\frac{y-19}{x-0}= \frac{1-19}{3-0}= \frac{-18}{3}=-6[/tex]
[tex]y-19=-6x[/tex]
[tex]\therefore y=-6x+19[/tex]
Thus, the complete graph will be a straight line [tex]y=-6x+19[/tex] as shown in the second diagram.
