The freshmen will have cost y = 100 + 50x (where x is the number of rolls). The sophomores will have y = 250 + 25x.
To graph these equations, which are already in slope-intercept form:
For the freshmen, we begin with the y-intercept at (0, 100). Then for each unit of x (to the right), we raise the y-value by 50. So we can put another point at (1, 150), at (2, 200), at (3,250), and so on. Any one other point is sufficient to draw the line.
For the sophomore, we begin with the y-intercept at (0, 250), and increase y by 25 for each unit of x. So we can connect it with (1, 275), (2, 300), and so on.
If both lines are drawn, we will see the intersection point at (6, 400), which means that both groups have a cost of y = $400 at x = 6 rolls.
