1. C ...the angles in the first statement add to 180. The angels in the second statement add to 180. One angle is used in each of these two equations, PTQ. SO, for PTQ + Angle 1 =180 and PTQ+Angle 2=180, it stands to reason that Angle 1=Angle 2
2. A. You are substituting AD for AC.
3.C. Substitution works. It seems the ideal answer isn't here. Substitution makes the most sense, given the initial info that BC=DC.