1)
What does he answer?
1.
Malbim: Your opinion is wrong. It would be correct if Avodah was for Hashem's need. Elifaz said "ha'Enosh me'Eloka Yitzdak" (4:17) - man must serve Hashem, for He created him. Hashem is unlimited, and so is His Avodah; a small sin is considered great. This implies that Hashem commanded Avodah for His need, therefore He must give reward and punishment, like a mortal king. You claimed, why were you not paid for your complete Avodah? You concluded that man's deeds are forced. He deserves reward no more than a mill that grinds due to the water that makes it turn. You err!
2)
Why did he say "v'Es Re'echa Imach"?
1.
Rashi: They were silent to your words.
