Hi Cain,

I would first express the depth of the liquid in feet rather than inches. There are 12 inches is a foot so 1 inch is one twelfth of a foot and thus one half an inch is $\large \frac{1}{24}$ of a foot. Hence the volume of liquid in cubic feet is the number of square feet of surface area times $\large \frac{1}{24}$ feet.

But you also know that the volume of liquid is 2.5 gallons. To express this is cubic feet I would ask Google. Type 2.5 gallons in square feet into the Google search window. Finally the surface area of the liquid in square feet is the volume of liquid in cubic feet, as supplied by Google, times 24 feet.

I hope this helps,
Penny