Answer:
 a)   
 
 b) 
 c) They'd have lost $1000 if they had sold no calendars.
 
Step-by-step explanation:
 a) The equation of the line in Slope-Intercept form is:
 
 Where "m" is the slope and "b" is the y-intercept.
 
 In this case we know that "y" represents the profit of loss and "x" the number of calendars sold. 
 Then, according to the exercise, the line passes through these two points:
  and
 and 
 Then, we can find the slope of the line with the formula 
 
 
 Now, we can substitute the slope and one of those points into  and solve for "b":
 and solve for "b":
 
 Then, subtituting values, we get that the equation that describes the relation between the profit of loss and the number of calendars sold, is:
  
 
 b) The slope of the line is the profit they made from selling each calendar
 
 c) The y-intercept is the amount they would have lost if they had sold no calendars:
 
 They'd have lost $1000 if they had sold no calendars.