Mark a distance 6 units from a fixed point A on a given line to obtain a point B such that AB=6 units.From B mark a distance of 1 unit and mark the new point C.Find the mid-point of AC and mark the point as O.Draw a semicircle with centre O and radius OC.Draw a line perpendicular to AC passing through B and intersecting the semicircle at D .Then BD=√6