We have 2 posts of 12m high each. In those posts there is a rope that joins them at their upper ends. This rope is 20m long.
If the lowest point of the rope is 2m from the ground, How far is it between the two posts?
The two posts have to be together because if the rope reaches 2m from the ground it means that it hangs vertically:
2 × (12 - 2) = 20