Answer
$(x^3+2x^2-6) \text{ miles}$
Work Step by Step
Using $A=lw$ or the formula for the area of a rectangle, then
\begin{array}{l}\require{cancel}
A=lw
\\
x^4-x^3-6x^2-6x+18=l(x-3)
\\
\dfrac{x^4-x^3-6x^2-6x+18}{x-3}=l
.\end{array}
Using the long division method below, then
\begin{array}{l}\require{cancel}
l=\dfrac{x^4-x^3-6x^2-6x+18}{x-3}
\\\\
l=x^3+2x^2-6
.\end{array}
Hence, the length, $l,$ is $
(x^3+2x^2-6) \text{ miles}
.$