Marco rented a small moving van. the moving company charges $40 a day for up to 5 days. the input is the time , in days, and the output is the total cost. this mapping diagram shows the inputs and outputs of the rental cost .

Input is time, this is your x-values
output is the total cost, this is your y-values. 
in the question above "this mapping diagram shows" so i'm not sure if there more to the question or what the question actually is, but if you have to create a mapping, you will create a list of x values for days and y values for corresponding total cost.
Day 1     cost $40    (1,40)   
Day 2     cost $80    (2, 80)    ($40 per day for 2 days = 40 (2) =  $80
Day 3     cost $120  (3, 120)  ($40 per day for 3 days = 40(3) = $120

hope this answers your question.  :-) 

If i'm correct the answer is $15.97

Horizontal change or the change of x