I need help finding the sum, please I've been doing this one question for over an hour

How to simply and write the answer n a whole number

Which geometric principle is used to justify the construction below?

The construction of copying qpr is started below. the next step is to set the width of the compass to the length of ab. how does this step ensure that a new angle will be congruent to the original angle? by using compass take the measures of angle and draw the same arc according to it.