she filled up her garden with 8ft³, so the volume of the garden is 8
the "depth" or height is 3/4, and the width is 2
[tex]\bf \textit{volume of a rectangular prism}\\\\
V=Lwh\qquad
\begin{cases}
w=2\\
h=\frac{3}{4}\\
V=8
\end{cases}\implies 8=L\cdot 2\cdot \cfrac{3}{4}[/tex]
solve for "L"
