**Problem VI:** To a given straight line AB and at a given point C in that line, to construct an angle given ACI equal to a given rectilineal angle E.

Construction: Construct above DE by the 4th problem, a perpendicular DF, meeting EF at F, and put at the point C in AB a line CG equal to ED, and from G construct over AB, by the preceding problem a perpendicular GH: I say that GI equals DF, and joining CI, the angle ACI will equal the angle E.