1. The problem asks what must be stated first when using the Hypotenuse-Leg (HL) theorem in a proof. 2. The HL theorem applies to right triangles and states that if the hypotenuse and one leg of one right triangle are congruent to the hypotenuse and one leg of another right triangle, then the triangles are congruent. 3. Before applying HL, you must first establish that the triangles involved are right triangles because HL only applies to right triangles. 4. This means the correct first step is to state that there are right triangles. 5. Other options like the reflexive property, CPCTC (Corresponding Parts of Congruent Triangles are Congruent), or vertical angles being congruent come after establishing the triangles and applying HL. Final answer: You must first state that there are right triangles.