Paul uses a rhetorical question to introduce the next topic he wants to discuss. It can also be translated as a statement. Alternate translation: "I will tell you what the purpose of the law is." or "Let me tell you why God gave the law." (See: [[rc://en/ta/man/jit/figs-rquestion]])
The words "by the hand of" are a metonym for the intermediary himself. This can be stated in active form. Alternate translation: "God ordained the law with the help of angels, and a mediator put it into force" (See: rc://en/ta/man/jit/figs-metonymy and rc://en/ta/man/jit/figs-activepassive)