Question 900898
Think of this like graph paper, where the trees each take up a spot.  The number of trees are the length of a row (L).  The number of rows is the width (W).
A = L * W
L = 2W-2
A = 84
What are the factors of 84?  Which pair fits the given equation?
Only 12 and 7
There are 7 rows of 12 trees