5.8375 is how many times 80 goes into 467
The coordinates of all the vertices:
A = (-7,5)
B = (-5,8)
C = (-2,4)
divide total paid by number of pencils for price of 1 pencil
4.50 / 18 = 0.25 for 1
so you would put a dot at (1, 0.25) (2,0.5) (3,0.75) etc and draw a line through the dots
see attached picture: