marginal benefit

F.B

i know this is not exactly where you post help questions but i really need it.
heres my questions.

I just started learning microeconomics and i am already confused. I was reading production possibility frontiers and how marginal cost and marginal benefit are related. I know that marginal cost is the slope of the PPF, and by doing this i get graph a marginal cost curve, but how do i calculate marginal benefits. I'll put some numbers.

Point A is (0, 15) and Point B is (1,14)
Marginal cost= 14-15/1-0 = -1

Now how do i find marginal benefit at these two points. I am trying to draw the marginal benefit curve.
If you need more points, then tell me and i will post them.

gravenewworld

Are you given a utility function? or a table with any utility data for a given amount of consumption?