A wheel of diameter ‘d’ is rolling without slipping with speed v on a horizontal road. When it is at point ‘A’ on the road a small lump of mud separates from the wheel  at its highest point  ‘B’ and drops at point  ‘C’  on the road . The distance ‘AC’ will be $v\sqrt{\frac{xd}{g}}$   then $x=\_\_\_\_\_\_$