Okay
W = [21/2 - sqrt(441/4 - 48)]/2
L = [21/2 + sqrt(441/4 - 48)]/2
When you add them and double the result, they give exactly 21. When you multiply them they give exactly 12.
I specifically said the problem became well-posed with answer B if you added the assumption that the lenght and width were rational numbers. Lengths usually aren’t: remember the Pythagorean theorem? A right triangle with legs of length 1 has a hypotenuse of length sqrt(2), which can’t be written as a fraction, and can’t be written in finitely many decimal places. If pressed I could give a compass and straight edge construction of the lengths of the sides needed to get the perimeter of exactly 21 and area exactly 12, but no one can give finite decimal approximations of them.
I multiplied and got 11.99952750039849