The park planning department should afforest the a b side of the river at equal distance, and both ends of the pixels in the previous one are in the known A B.

(1) From the nature of the midline, the distance from the point on the midline to the two endpoints of the line segment is equal, so the midline of AB intersects with L at point P, which is the required site;

② Make a symmetrical point E of point A about L, connect EB and L at point P, and connect AP, then point P is the site.