we know that
The volume of the tank is equal to

where
B is the area of the base of the tank
h is the deep of the tank
In this problem we have


Solve for B

substitute the values

-------> area of the base of the tank
we know that
the base of the tank is a square
the area of a square is

where
x is the length side of the square


therefore
the answer is
the length of each side of the base of the tank is
