"Why do they say ... son?" Jesus uses a question to make the scribes think about who the Messiah is. AT: "Let's think about them saying ... son." or "I will talk about them saying ... son" (See: [[rc://en/ta/man/translate/figs-rquestion]])
The prophets, the religious rulers, and the Jewish people in general knew that the Messiah was the son of David. AT: "everyone says" or "people say" (See: [[rc://en/ta/man/translate/figs-explicit]])
"King David's descendant." The word "son" is used here to refer to a descendant. In this case it refers to the one who would reign over God's kingdom. (See: [[rc://en/ta/man/translate/figs-synecdoche]])