Jeremiah's genuine prophecy is in verses 12 through 17.
After Hananiah had broken the yoke bar from the neck of Jeremiah, the word of Jehovah came to Jeremiah, telling him to go and speak to Hananiah (v. 12). Jeremiah was to say to him, "Thus says Jehovah, You have broken the wooden yoke bars, but you have made in their place iron yoke bars" (v. 13).
Jeremiah went on to say that Jehovah, the God of Israel, had put an iron yoke upon the neck of all these nations that they may serve Nebuchadnezzar, the king of Babylon, and they would serve him (v. 14).
Finally, Jeremiah said to Hananiah that Jehovah had not sent him and that he had made this people trust in falsehood (v. 15). He also told him that he would die that very year because he had spoken rebellion against Jehovah. This prophecy was soon fulfilled, for the prophet Hananiah did die in that very year in the seventh month (vv. 16-17). This should have helped all the people to discern who was genuine and who was false.