Question 517066
{{{lw=A}}}
{{{w=l-6}}}
{{{A=40}}}
l and w must be positive
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{{{l(l-6)=40}}}
{{{l^2-6l=40}}}
{{{l^2-6l-40=0}}} 
find 2 numbers that multiply to get -40 and add to get -6
-10(4)=-40 ,  -10+4=6  so -10 and 4
{{{(l-10)(l+4)}}}
{{{highlight(l=10)}}} cannot be negative
{{{w=l-6}}}
{{{w=(10)-6}}}
{{{highlight(w=4)}}}