Question 284109
{{{A = area = Length  * Width}}}
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{{{A = x^2 + 3x -18}}}
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{{{W = width = x-3}}}
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Since
{{{A = L*W}}}
We can rearrange to say that
{{{A/W = L}}}, 
which suggests you divide A/W (that is, polynomial division).
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But you may be able to factor the equation to advantage.
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{{{x^2 + 3x -18 = (x+6)(x-3)}}}
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So substituting what we know
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{{{A = L * W}}}
{{{A = (x+6) * W}}}
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Which means 
{{{L = (x+6)}}}
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Done