We can call x the number of candy bars sold. The linear equation for the fund can be represented by y=150+3x dollars (this is the rule). That's the initial amount of $150 plus $3 for each candy bar. When y=$237, the goal, 237=150+3x, so 3x=237-150=87 and x=87/3=29 to meet the goal. A graph of y=150+3x can be drawn where 150 is the y intercept and -50 is the x intercept. By joining these intercepts and continuing to the right you will see a graph showing values of x and corresponding values of y on the graph to the right. The graph can be drawn near the bottom of the page to allow y to extend to about 240 with suitable scaling. The only reason to draw the graph away from the left of the page is to accommodate x=-50, the x intercept. The only meaningful part of the graph is to the right of the y axis and above the x axis, the positive region for both x and y. If you read off the x value for y=237 you should see that it's 29. You can also see what value of y corresponds to x=14. It should be y=192. A comfortable scaling for both axes is tenth of an inch for each unit, so 80 units on the x axis is 8" and 240 units on the y axis is 2 feet (a long sheet of paper!). To shorten the y axis, you can take y from 150 to 240, 90 units, or 9". That means losing 5" on the x axis so that x starts at 0 and goes up to about 30 or 3". You don't need the same scale on each axis, so you could divide the y axis so that 1/10=2 units, making the y axis 12" instead of 24". Play around with the scaling and show your daughter what you are doing. To make a table of x and y values, for example:

x y

0 150

10 180

20 210

30 240

Join these points to get the line and choose the scale that gives you the best readings.

I hope this helps you both.