Question 835344
The discussion you give seems to show interest on just the floor area and dimensions.


A=L*W and you are given A=418.
Not given which is which, one dimension is the other +3.  We could choose W=L+3 but no harm either way.


You now just have:
L*W=418 and W=L+3
You have enough to do a substitution to obtain an equation in one single variable.   See how?   You started and then became stuck.  No reason for being stuck.


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{{{L*W=418}}}
{{{L*(L+3)=418}}}
{{{L^2+3L=418}}}
{{{L^2+3L-418=0}}} ----- Try to factor this if you want, but more comfortable just using general solution to quadratic formula.  Not sure which course you're in.  If intermediate or higher, then you know what to do.  If Intro or Elementary, then you would probably be expected to factor.
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FACTOR?
418=2*209=2*11*19----- maybe we have a way to factor.
22 and 19?
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{{{highlight((L+22)(L-19)=0)}}}
TWO SOLUTIONS.  {{{highlight(cross(L= -22))}}} OR  {{{highlight(L=19)}}}.
So the length, L can be 19 feet, and therefore W for width can be 19+3=22 feet.
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SUMMARY: Length 19 feet, Width 22 feet
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