Rashi: [Strengthen yourself] in Derech Eretz
1
, like it says "you will cause Yisrael to inherit."
2.
Malbim: Chazak is to arouse your strength. Imutz is strong resolve in the heart to persist in this. Divine help is according to a person's preparation. Since you will cause Yisrael to inherit, you must invigorate yourself that no one will oppose you to have quarrels.
Rashi (Brachos 32b): If he is a craftsman, Derech Eretz is his craft. If he is a merchant, it is his business. If he is a warrior, it is his war.
2)
Why did Hashem mention the Shevu'ah to the Avos?
1.
Malbim: It is a third reason why he should not fear. (a) You will make them inherit, and no one else. (b) It is the Am (chosen nation) that will inherit. (c) I swore to the Avos to give it
