Answer: 70 meters.
Step-by-step explanation:
Observe the figure attached.
The distance between the walls is:

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Both triangles are right triangles. Therefore, you can calculate the distance between the walls as following:
- Calculate the distance AD:

- Calculate the distance AE:

Therefore the distance between the walls is:

3 ajjajajajajajajajjaajja
7x9=63
63÷2= 31.5x4
126= area of the triangles
7x7= 49= area of the square.
126+49= 175 ft