What is the meaning of "va'Yavo Alav"?
Rashi: Achaz sent to Melech Ashur to help him, like it says in Melachim II, 16:9.
Why does it say "va'Yatzar Lo v'Lo Chezko"?
Rashi: Achaz was pained that he gave to him the money of Beis Hashem and Beis ha'Melech, and Melech Ashur did not help him.
Radak #1: Melech Ashur afflicted him, and did not strengthen him. Chazako is Po'el Yotzei (acts on another), like "Lasum Chitul Lechavshah Lechazkah" (Yechezkel 30:21). Since Achaz sent [a bribe] for him to help him, it says not only did he not help him, rather, he came and afflicted him!
Radak #2: It is Po'el Omed. Achaz was not strengthened via Melech Ashur. Rather, he also [afflicted] him, after he destroyed Damesek and Shomron.
