Let
P-------> the number of pages Margaret has left to read 
d-------> the number of days Margaret has been reading
we know that

This is a linear equation
where
 ------> is the slope of the equation
 ------> is the slope of the equation
represent 

 ------> the total number of pages in the book
 ------> the total number of pages in the book
 ------> the number of pages she has read during d days
 ------> the number of pages she has read during d days
the graph in the attached figure