Question 861680
Common enough to be worth generalizing.

width w, length L.
Width is soe k feet shorter than length, or w=L-k.
Area is given, and may be called A.
The unknown variables are L and w.
(A=12 and k=4).


{{{wL=A}}}
{{{(L-k)L=A}}}
{{{L^2-kL-A=0}}}
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Note that your given values for area and for k may render the quadratic equation in L as factorable.  In GENERAL, you cannot rely on factorizability.
General Solution to Quadratic Equation gives for L,
{{{L=(k+sqrt(k^2-4(-A)))/2}}}
{{{highlight(highlight_green(L=(k+sqrt(k^2+4A))/2))}}}.


You can substitute the values for A and k to compute L.