Final answer:
The constant of proportionality for a proportional relationship between y and z is approximately 2.
Step-by-step explanation:
The constant of proportionality for a proportional relationship is found by dividing the value of y by the value of z. In this case, the graph shows a proportional relationship between y and z. To find the constant of proportionality, we select any point on the graph and calculate the ratio of y to z at that point. Since the graph does not provide specific points, we can estimate the ratio based on the graph. Looking at the graph, it appears that the ratio of y to z is approximately 2. Therefore, the constant of proportionality y / z is approximately 2.