I have 2 zf7762 install at both side for the wall facing an rectangular area of 13.75m width and 17.6m long. both AP mount on left and right of the wall with no obstacle, 8m away from each other. what is the recommend height of the ap installation if it is directly mount to the wall?
At that distance just about any height will work. Generally you will probably want the APs to be about ceiling height, about 10-15 feet high and the APs should be at the same elevation, assuming dome down configuration if you need the APs to be able to MESH together, if not it should not matter, put in such a way to provide the best coverage to your users, which is about 10-15 feet above user regardless of the other AP.
Hope that helps, but this is a location/application specific question, so can't be easily answered.