Chapter 6 - Polygons and Quadrilaterals - 6-4 Properties of Rhombuses, Rectangles, and Squares - Practice and Problem-Solving Exercises - Page 382: 58

The first step of the indirect proof should state, "$\triangle PQR$ is a right triangle."

The first step to writing an indirect proof is to assume that the conclusion is not true. Therefore, the first step of the indirect proof should state, "$\triangle PQR$ is a right triangle." The rest of the proof tries to prove that the triangle is a right triangle, but when it doesn't, then we have to assume that the triangle isn't a right triangle after all.