Solve the proportion

In the diagram of circle o, what is the measure of zabc?

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.

A+b= 17, ab=70, find the value of |a-b|