Junior in chemical engineering here also vouching for how rad khanacademy.org is. Definitely look into it for loads of video tutorials and great little mini-lessons. Calc is relatively fresh in my brain as I just finished calc 4 (diff. eq, some universities call that calc 3), and I might be able to help you out with some stuff. Not on NS much, but maybe we both might get lucky and I can help you solve something.
As far as seeing relevance to marketing or economics, many things can be mapped into functions. Profit can be made into a function dependant on time for example when looking at the ebb and flow of a year's profits. Calculus can be (sort of) beneficial in analyzing maximum profit rates (which would be the slope or derivative of the original function) or finding critical points or whatever as well. These are obvious by just looking at the graph in something as simple as profits vs. time, but I'm sure it can be useful in other ways. Plus, I found taking calc taught me a whole new strategy for problem solving, which has helped me in other courses. Yeah it might be "useless," but take what you can from it.