Hey!! I'm not Expert/Ace, but I am a Virtuoso so I hope I can help :p
Braden bought 18 pencils for $4.50
The number of pencils bought is x, and total cost is y.
x = 18 y = 4.5
you would then plot the point (18, 4.5).
I am assuming that because you need a line, I will need to make another pencils bought/price.
The point I made is (30, 7.5).
(You can get another point by dividing 4.5 by 18, and then using the result, .25 to see how much it would cost to by x amount of pencils.)
Attached is the line I got by plotting both points, and connecting them :)
I hope I answered your question!!