4. A vertical wall of height 27 metres stands parallel to a tall building. The wall is 64 metres from the tall building. A ladder with its foot on the ground is to pass over the vertical wall and touch the tall building. Determine the length of the shortest possible ladder that can do this. Use a diagram to support your solution. Round your answer to 1 decimal place. Note: You must use trigonometric differentiation. [5 Marks]