how to get it?

A post 20 meters long stands vertically in a horizontal plane. Another post stands vertically on the same horizontal plane, 45 meters high, and 80 meters from the first post. A peg must be placed on the horizontal plane that will hold the rope from the top of the first post to the peg then to the top of the second post. How farfrom the shorter pole must the peg be installed so that the length of the rope is minimum