My homework question says " what is the opportunity cost of good x in term of y, when production is shifted from point b to c along the pcc?"

For b the x axis is on 40 and the y axis is at 75.
So the opportunity cost will be 35

For c the x axis is 70 and the y axis is 50
So the opportunity cost will be 20

So do I combine them ?subtract them? or leave them as it is?

To determine the opportunity cost of good x in terms of y when production is shifted from point b to c along the production possibility curve (PCC), you should calculate the difference between the two opportunity costs.

In this case, the opportunity cost of good x at point b is 35, and the opportunity cost of good x at point c is 20. To find the change in opportunity cost, subtract the opportunity cost of c from the opportunity cost of b:

Change in Opportunity Cost = Opportunity Cost of b - Opportunity Cost of c

Substituting the values:

Change in Opportunity Cost = 35 - 20

Therefore, the change in opportunity cost is 15 units.