Depends on which maoyi you mean. Why don't you write in Chinese? 贸易 commercial trade 毛衣 (woollen) pullover So it is either: 1. 他要讲关于贸易方面的问题 。He wants to talk about trade related issues. (As Zhang the Great said above [love that name]) or 2. 他要讲关于毛衣方面的问题。 He wants to talk about problems associated with woollen pullovers. (Maybe allergies, discomfort, the need to wash carefully) I'd choose 1. but 2. is a possibility.

他要讲关于毛衣方面的问题 He is talking about the pros about sweaters

他/她要讲关于贸易方面的问题。 He/She wants to discuss trade-related issues.