You can put this solution on YOUR website! First, find the volume of the cylindrical tank: where r = the radius of the circular cross-section and h = the height (or depth) of the tank.
The cylinder cross-section has a diameter of 54" so its radius is inches. The height (depth) is 27". cubic inches.
Since you didn't say what liquid is in the tank, I'll assume that it's water and, for water, 1 gallon = 231 cubic inches.
So you need to know how many times will 231 cubic inches go into 61835.97 cubic inches, so you have to divide: gallons.