Why would Pharaoh listen to god?

Exodus 3:18 says, “And they shall hearken to thy voice: and thou shalt come, thou and the elders of Israel, unto the king of Egypt, and ye shall say unto him, The LORD God of the Hebrews hath met with us: and now let us go, we beseech thee, three days' journey into the wilderness, that we may sacrifice to the LORD our God.

3:19 And I am sure that the king of Egypt will not let you go, no, not by a mighty hand.”  Why would Pharaoh, a god, listen to a bunch of Hebrews, let alone let them go out and worship in the wilderness.  Letting them leave defeats the idea that they were slaves, let alone the fact that he probably wouldn’t understand why they wouldn’t want to worship him?  He is a god and right there in front of them, after all.