Explanation:[br]Let there exist line AB and angle CDE.[br]Let C and E be chosen be taken at random and CE joined.[br]Pick pt F on AB such that AF =DE. [br]let AG= CD and FG= CE. (Using circle radii and the proposition before this one in the book)[br]By SSS, these triangles are congruent.[br]Thus, Angle FAG = angle DCE.[br]Therefore, a rectilineal angle on a line had been constructed such that it is = to another rectilineal angle.