There is no unique solution to this but a straight line can be put through the data as follows:

Put:

c = m p + k

Now when p=2 c=120, so we have:

120 = 2 m + k

and when p=3 we have c=60, so we also have:

60 = 3 m + k

subtract these, to get: 60 = - m or m=-60, and now put this back into the second

to get:

60 = 3(-60) + k

so k = 240.

RonL