You could do what my friend in ROTC did and tell your prof that a failing grade would result in you getting shipped to Iraq to die.
You're going to have trouble convincing the professor if you haven't been to see him at all thus so far in the semester, and if it's after the last exam, I have no idea how you feel you could get any sympathy. However, the best thing to do would just be to lay it out and say something like "I haven't had an easy time with the material and the first exam really caught me off guard, but I've been working hard all semester and I really want to get at least a C-. What do I need to do to help out my average?"
“It is a strange paradox that today’s central banks are generally staffed by economists, who by and large profess a belief in a theory which says that their jobs are, at the best, unnecessary, and more likely wealth-destroying. Needless to say, this is not a point widely discussed among respectable economists. Nevertheless, it is an issue worth pondering.”
George Cooper, The Origin of Economic Crises