The width of a rectangle is 3/4 its length.
[tex]w=\frac{3}{4}l[/tex]The perimeter of the rectangle is 420 ft.
Recall that the perimeter of a rectangle is given by
[tex]P=2(w+l)[/tex]Let us substitute the value of the given perimeter and the width
[tex]\begin{gathered} P=2(w+l) \\ 420=2(\frac{3}{4}l+l) \end{gathered}[/tex]Now simplify and solve for length
[tex]\begin{gathered} 420=2(\frac{3}{4}l+l) \\ 420=\frac{3}{2}l+2l \\ 420=3.5l \\ l=\frac{420}{3.5} \\ l=120\: ft \end{gathered}[/tex]Therefore, the length of the rectangle is 120 feet.
