Abcdefgh is a regular octagon. the minimum degree of rotation by which this octagon can map onto itself is °. it will take increments of that degree of clockwise rotation for a′ (the image of a) to coincide with c.

Imagine putting tracing paper on top of the octagon and tracing out the shape. If you now rotate the tracing paper about the centre, you would have to go approximately 360/8=45 degrees until the traced shape matched the octagon underneath. Assuming A,B and C are lines of the octagon, you would have to rotate by 45 degrees twice clockwise for the image of A on the tracing paper to conicide with C.