1)
To what Chachmah should he hearken?
1.
Malbim: The laws of Chachmah. There is no proof for them; they are received from tradition.
2)
Why does it say "Hat Aznecha"?
1.
R. Yonah: He already commanded against company of the Rasha. He commands the ear to hear Divrei Chachamim.
2.
Malbim: Tevunah is what a person understands by himself, and derives a matter from a matter via his Binah. Weigh it with the scale of your Binah!