The y intercept is always in the back as it indicates where to start your line. In this case at 5 on the y axis. then the slope with the as it indicates how the line the of the graph will go. Hope this helps.
If she uses 2 bags of soil for 6 plants, that would mean that she is using 1 bag per 3 plants. Therefore if she was planting 15 plants, she would use 5 bags of soil.
