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Since the density is obviously 2 seedlings/hill, I suspect you want the density in terms of seedlings/m^2 or something.
Since each hill occupies 400cm^2, the density is
2 seedlings/400cm^2 = 1/200 seedlings/cm^2
Maybe not too useful. In terms of m^2, that will come out
2 seedlings /(.20m)^2 = 2/.4 = 5 seedlings/m^2
The size of the garden and number of rows makes no difference, unless you have some space left unused, and you want the exact density for the garden plot.