I am assuming having no slope would be when a line is undefined, that would mean it is a vertical line <span>a line with 0 slope would actually be a straight horizontal line.</span>
So you do 20%*426 to get 85.2 then you would subtract 85.2 from 426 because they sold 20% more than last year. On top of that the answer isn't 355 it is 340.8.