+Sakib Sadman if you assume k = 1 then the equation simplifies to 2x = 0...meaning x = 0. So, if y=mx + b (the equation of a line) and the slope is zero...what type of line is that? That is a vertical line, which is not represented by any of the graphs. Basically, they are helping you by limiting what types of graphs could result from the original equation.
+Yabo Detchou did you click on the link in the window below the video? click the link and navigate the pdf document to page 152...that's where all of the questions come from
+Amar Babic that's b/c of my awesome tutoring! hehe, well I just did the ones that college board offered through their website, when I find some tough ones i'll be sure to post them,gl