Final answer:
In this proof, step 3 involves showing that TR is parallel to AP. The given slopes of TR and AP allow us to conclude that TR and AP are parallel, making the quadrilateral TRAP a trapezoid.
Step-by-step explanation:
Step 3 to complete the proof is: tr || ap by the definition of parallel lines. The definition of a trapezoid states that in a trapezoid, one pair of opposite sides is parallel. From the given slopes, we can see that the slope of TR is 3 and the slope of AP is -3/4. Since the slopes are equal, we can conclude that TR and AP are parallel, making the quadrilateral TRAP a trapezoid.