Chapter 2 - Reasoning and Proof - 2-3 Biconditionals and Definitions - Practice and Problem-Solving Exercises - Page 101: 18

If $x^2=144$, then $x=12$ or $x=-12$. If $x=12$ or $x=-12$, then $x^2=144$..

The phrases on either side of "if and only if" in a biconditional can be used as the hypothesis and conclusion of a conditional and its converse. Recall that the converse of a conditional switches the hypothesis and conclusion.