If LM=x^2, MN=x, and LN=12, then
x^2 + x = 12
Manipulative into quadratic form.
x^2 + x  12 = 0
Factor products of trinomal.
(x + 4) (x  3) = 0
Solve.
x = 4 or x = 3
Eliminate negative length.
x = 3
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