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l in (-oo:+oo)
168 = (21/2)*(l-11) // - (21/2)*(l-11)
168-((21/2)*(l-11)) = 0
(-21/2)*(l-11)+168 = 0
168-21/2*(l-11) = 0
168-21/2*(l-11) = 0
567/2-21/2*l = 0
567/2-21/2*l = 0
567/2-21/2*l = 0 // - 567/2
-21/2*l = -567/2 // : -21/2
l = -567/2/(-21/2)
l = 27
l = 27
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