Arex must admit that what Grecko was telling him was the truth. Grecko could as well feel that he has potential and that's why he wanted Arex to improve for the better.
After contemplating on the things that previously took place, particularly the fight between him and Grecko, Arex, despite how he didn't want it, eventually had to accept that such truth would eventually help him to become stronger, even if it would mean that he would have to agree with Grecko.
"Fine, so how do I become strong?" Arex eventually asked, curiosity was painted all over his face.
And as he anticipated Grecko's response, he then witness how Grecko once more manipulated his orb to male the emerald flame grew bigger.