A METZORA MUST SHAVE WITH A TA'AR
Answer #2: If he fulfills his Mitzvah with a Melaket or Rehitani, since the Torah did not specify Ta'ar, Reish Lakish's law would forbid him to use a Ta'ar! (This argues with Answer #1.)
(Reish Lakish): Wherever one can fulfill the Aseh and the Lav, he must. If not, the Aseh overrides the Lav.
Question: How does R. Eliezer learn that a Metzora shaves with a Ta'ar?
Answer: He learns from "Rosho":
(Beraisa) Suggestion: Perhaps "A Ta'ar will not pass over Rosho (of a Nazir)" applies even if he is a Metzora!
Rejection: "(He shaves) Rosho" (even if he is a Nazir).
Question: Perhaps a Metzora can fulfill his Mitzvah also with a Melaket or Rehitani!
"Rosho" comes to teach that he is allowed to shave with a Ta'ar.
One would have thought that a Nazir is liable for shaving with a Ta'ar, even when he is a Metzora. The verse teaches that this is not so.
Answer: If he fulfills his Mitzvah with a Melaket or Rehitani, since the Torah did not specify Ta'ar - Reish Lakish's law would teach that he may not use a Ta'ar!
HOW THE OTHER VERSE IS EXPOUNDED
Question: What do Chachamim learn from "Rosho"?
Answer: It teaches that Tiglachas Metzora overrides the Lav of cutting the Pe'os.
(Beraisa) Suggestion: Perhaps "Lo Sakifu Pe'as Roshchem (do not round the corners of your heads, i.e. cutting the Pe'os)" applies even to a Metzora!
Rejection: "(A Metzora shaves) Rosho."
Question: Why did the Torah need to write "Rosho"? We could learn from "Zekano" (that an Aseh overrides a Lav)!
(Beraisa) Suggestion: Perhaps "(Kohanim) will not shave the corners of their beards" applies even to a Metzora!
Rejection: "(A Metzora will shave) Zekano" (even if he is a Kohen).
Summation of question: Why are both "Rosho" and "Zekano" needed?
Answer: Had the Torah written only "Zekano", one might have thought that Hakafah (the Isur to cut the Pe'os) does not apply when one cuts all his hair. Therefore, it says "Rosho";
Had the Torah written only "Rosho", we would have learned two laws.
An Aseh overrides a Lav;
Hakafah applies when one cuts all his hair.
We would not learn that a Metzora must shave with a Ta'ar. Therefore, "Zekano" is also needed.
Question: What is R. Eliezer's source that an Aseh overrides a Lav?
Answer: He learns from Tzitzis.
(Beraisa): "Do not wear Sha'atnez", but "Gedilim Ta'aseh Lecha" (you may use Sha'atnez to fulfill the Mitzvah of Tzitzis).