x is time and y is pencils that you have
'starts with 90' so at x=0, plot y=90
'gives away 15 every hour' so for every 1 hour, move 15 down on y
so at x=1, do 90-15=75 pencils
at x=2, do 75-15=60 pencils
etc
until you get to
x=6, do 15-15=0 pencils then stop because you can't give away 15 pencils when you have 0
see attachment for plot
I don't know how your segment tool works so you do it
if it draws a line, you only need 2 points, just do when x=0, y=90 and when x=6, y=0
