A) The term "right angle" is another way of saying "90 degree angle". This is a definition. Think of a definition in a dictionary.
B) The substitution property allows us to replace one thing for another, as long as the two things are equal. The transitive property works as a similar idea.
C) If two angles have the same measure, then they are congruent. This is the definition of congruence (or one form of it).
The trick to finding the value that completes the square, is dividing the middle number, the 10, by 2, and squaring that. Thus, we have 10/2=5, and 5^2=25.
