Question 6257
You could do this in three steps.

1) You could find the volume of the pool in cubic feet, if only you knew how long the pool was! Because finding the volume of a rectangular prism, the usual shape of a swimming pool, requires that you multiply the length by the width  by the depth. You have provided the depth (8 ft.) and the width (48 in or 4 ft.). So, what's the length of the pool?

2) Once you have found the volume, you will need to find the density of water in U.S. gallons per cubic foot. This is 7.48 gallons per cubic foot.

3) Finally, you would multiply the volume of the pool (in cubic feet) by the density of water (gallons per cubic foot) to find the number of gallons of water held by the filled swimming pool.