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?