An exterior architect is asked by a builder to plant seven trees in a manner that there are exactly six rows of trees in a straight line and each row has three trees in particular.

How will he do it?

He will plant three trees as the vertices of an equilateral triangle. Now imagine an equilateral triangle. Three more trees will be planted on the center of each of the side of the triangle. The remaining one tree will be planted at the centroid of the hypothetical equilateral triangle.