1)
What is "Michtav"?
1.
Rashi citing Targum Yonasan: He wrote thanks for the miracle done for him. Radak - this was when he was cured. It was a remembrance of the Chesed that Hashem did for him, to add to his days 1 .
2.
Ba'al ha'Turim: The following is one of 10 Shiros (the last will be in the future). The Yud in "[Az] Yashir" (Shemos 15:1) hints to (its Gematriya is) 10.
1
Radak: I explained adding days in Sefer Shmuel and Melachim. Refer to Melachim II, 20:6:1:1,2, refer to Shmuel I, 26:10:5:1 and the note there.
2)
What is the meaning of "va'Ychi"?
1.
Radak: He was healed. This is like "Ad Chayosam" (Yehoshua 5:8).
