1)
What land remained?
1.
Rashi: It is from what was promised to Avraham, and was not conquered.
2)
Why does it say "Nish'arah", and not Nosrah?
1.
Malbim: Nish'arah implies intentionally. Yehoshua was lazy about conquering everything, for he thought that he will not die until he conquers it all.
