A. You can use y=mx+b to find the rate of change of a bushel corn in the current year. The rate of change would be y=8x.
B. One bushel is worth 8$ (24/3 = 8) in the current year, and in the previous year one bushel was worth 7$ (21/3= 7). Therefore there was a 1$ raise from the previous year to the current year.