What branch of mathematics deals with equations with variables?

Need this asap if anyone can i would be very grateful. if you could show workings that would really

Consider the proof. given: segment ab is parallel to line de. prove: what is the missing statement in step 5?

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.