Final answer:
To incorporate a fourth tree into the design as the vertex of a parallelogram with the three existing trees, the landscape architect should place it at the intersection of lines parallel to the sides formed by the existing trees.
Step-by-step explanation:
The landscape architect must place the fourth tree such that all four trees form the vertices of a parallelogram. To achieve this, the architect should identify the existing three trees as three vertices. By drawing two lines parallel to the lines joining these trees, the intersection of the two lines will determine the possible locations for the fourth tree. If trees A, B, and C are the existing trees, with AB and BC forming two sides of the parallelogram, then the line parallel to AB starting from C and the line parallel to BC starting from A will intersect at the location for the fourth tree, D, completing the parallelogram ABCD.