so if the base is a square, which is 3'9" on a side, that means the tank is just a tall square, namely a rectangular prism.
since there are 12 inches in 1 foot, then 3 feet is 36 inches, and 3'9" is then 36" + 9" or 45 inches, so the base is a 45"x45" square.
now, let's covert the gallons to inches as well, namely in³, keeping in mind a (US) gallon has 231 in³, then 40 gallons will be 40*231 or 9240 in³.
[tex] \bf \textit{volume of a rectangular prism}\\\\
V=Lwh~~
\begin{cases}
L=length\\
w=width\\
h=height\\
-----\\
L=45\\
w=45\\
V=9240
\end{cases}\implies 9240=(45)(45)h
\\\\\\
\cfrac{9240}{(45)(45)}=h\implies \stackrel{inches}{2.025427}\approx h [/tex]
