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The number of rows is 3x and the number of trees in each row is 2x, according to the condition,
where x is the common measure (now unknown) of these two numbers.
In addition, we know that the total number of threes in the garden is 864:
(3x)*(2x) = 864.
It gives you
6x^2 = 864
x^2 = 864/6 = 144 ====> x = = 12.
Thus the length of each row is 7*(2x) = 7*2*12 = 168 yards.
The other dimension of the garden is 7*(3x) = 7*3*12 = 252 yards.
You may want to add a uniform strip around this rectangle of 7/2 = 3.5 yards wide.
It depends on conditions that are not defined clearly and explicitly in the problem, so I leave it up to you.