Answer:
, 
Step-by-step explanation:
The equations of volume and surface area are presented below:


The length of the tank is:

The expression fo the surface area is therefore simplified into an univariable form:


The first and second derivatives of the expression are, respectively:


The first derivative is equalized to zero and length of the square side is now found:



![l = \sqrt[3]{\frac{138}{4} }](https://tex.z-dn.net/?f=l%20%3D%20%5Csqrt%5B3%5D%7B%5Cfrac%7B138%7D%7B4%7D%20%7D)

Now, the second derivative offers a criteria to determine if solution leads to an absolute minimum:

(Absolute minimum)
The depth of the tank is:

