This solution took 2 pages, I will attach the 2nd page after this :)

This is the 2nd page. For the last part considering it is worth 5m, a considerable working is likely required (as see here) to obtain the answer. I started by letting the coordinates of D be 2 variables (a, b) and formed 2 equations to solve simultaneously. One through the gradient formula for 2 perpendicular lines and the area of half the kite. Hope this explanation helps :)