Question 763666
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Let length be L. Width is then L+3 cm. 

Area = {{{L*(L+3) = L^2 + 3*L = 40}}}

{{{L^2 + 3*L - 40 = 0}}}

Factorizing the left side

{{{(L + 8)*(L - 5) = 0}}}

L = -8 or L = 5

Since L cannot be negative, length = 5 cm and therefore Width = 8 cm.

Check: Area = L*W = 5*8 = 40. Correct!

:)
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