The answer asks for the units in square feet, and you are given inches, so you first need to convert your units to feet. There are 12 inches in a foot, so instead of 48 in x 24 in, this is 4 ft x 2 ft. (Divide each number by 12 in/ft.)
The final area is length x width, or 4 ft x 2 ft = 8 ft squared.