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]])
This can be stated in active form. Alternate translation: "God ordained the law through angels" or "God gave the law through angels" (See: [[rc://en/ta/man/jit/figs-activepassive]])
The words "by the hand of" are a metonym for the intermediary himself. An intermediary represents one person to another. Paul was probably talking about Moses, who represented God to the people of Israel. Alternate translation "by an intermediary" or "by his representative, Moses" (See: [[rc://en/ta/man/jit/figs-metonymy]])