a. The constant of proportionality is the ratio of the change in y to the change in x. From the information given, we can see that the change in y (income) is 70 - 0 = 70, and the change in x (hours) is 5 - 0 = 5. Therefore, the constant of proportionality is 70/5 = 14.
b. Since the relationship is proportional, we can write the equation as y = kx, where k is the constant of proportionality. From part a, we found that k = 14, so the equation that represents this graph is y = 14x.
c. Since the graph starts at y = 0 when x = 0, the y-intercept is 0.