Answer:
$8.40
Step-by-step explanation:
to find discount you use part=%(whole) so you are looking for your part but know the % and whole so you do x=40%(21) and 40%, or 0.4, times 21 is 8.4 so thats your part, 8.4 is 40% of 21 and thats how much youll pay
D is your answer
plug in the x value and y value to the equation
when x = 1, and y = 4
-1 + 4 = 3
when x = 5, and y = 8
-5 + 8 = 3
hope this helps :D