Ffor CD to be as short as possible <'s ACD and BDC must be right angles.
and therefore the points C and D will be the same height above the ground.
Using trigonometry the height of A above the ground = AO sin30
= 20*0.5 = 10 m, so the height of point C is 10+6 = 16 m above the ground
Height of B = 20* sin 45 = 14.142 m ,
So as C and D are both at the same height the length of BD = 16 - 14.142
= 1.86 m to nearest hundredth. Answer
Hope this helps