Can someone plz me solve this math problem. tnxxx : )

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