Answer:
24
Step-by-step explanation:
let [tex]\ell=length[/tex]
then [tex]width=\frac{3}{4}\ell[/tex]
since the perimeter is 84, then
[tex]2(length+width)=84\\ 2(\ell+\frac{3}{4}\ell)=84\\ 2(\frac{4}{4}\ell+\frac{3}{4}\ell)=84\\2(\frac{7}{4}\ell)=84\\ \frac{7}{2}\ell=84\\ \ell=\frac{84\times 2}{7}\\ \ell=24[/tex]
