In 9:12-13 Jesus speaks twice of what is written: "How is it written about the Son of Man" (9:12) and "even as it is written about him" (9:13). Where?

Mark doesn't say. They ask him, "Why do the scribes say that Elijah must come first?" (9:11). He answers, "Elijah indeed comes first, and restores all things. How is it written about the Son of Man, that he should suffer many things and be despised?" (9:12) (tap restores), and "But I tell you that Elijah has come, and they have also done to him whatever they wanted to, even as it is written about him" (9:13). Mark names no book and quotes no words, so these notes name none. Mark doesn't say whom Jesus meant by "Elijah has come" either. Matthew's telling ends, "Then the disciples understood that he spoke to them of John the Baptizer" (Matthew 17:13). Favor's chapter on 2 Kings 1 asks whether John was Elijah, with Malachi 4:5 and Matthew 17:12-13, and gives the main readings there; this chapter doesn't ask it again.