prepositions

Actually, they both work, although the use of "at" in such a context might be considered colloquial. To be on the safe side, i'd advise using "in." There is no hard and fast rule of which i know, but generally, "at" would be used in reference to a physical skill--you are poor at baseball. "In" would be used in reference to knowledge--you are weak in English. These are almost interchangeable, but it is difficult to answer, since it is one of the colloquial usages which one simply knows how to use, without thinking about the reason.