I understud clearly the exercise untill the part of page 121, where is part of Solve using is-contribution line: I am confused, where the number comes from, when its written: y= 800 , and after I have to put in contribution equation
They chose 800 because it would fit nicely on the graph.
You can choose any number that you like, because whatever line you end up with will be parallel and it is the angle (slope/gradient) of the line that is all that matters.