Step 1) convert everything into feet:
Windows: (already in feet) 2 ft x 3 ft
Door: 6 ft 8 inches = 6 + 8/12 = 8.66667 ft and 32 inches = 32/12 = 3.66667 ft
Walls: Front-to-back = 15 ft 6 inches = 15 + 6/12 = 15.5 ft and side-to-side = 13 ft 9 inches = 13 + 9/12 = 13.75 ft, and height = 96 inches = 96/12 = 8 ft
Step 2) Calculate area of walls and ceiling:
Wall 1: width x height = (15.5)(8) = 124 sq ft
Wall 2: width x height = (13.75)(8) = 110 sq ft
Wall 3: width x height = (15.5)(8) = 124 sq ft
Wall 4: width x height = (13.75)(8) = 110 sq ft
Ceiling: width x length = (15.5)(13.75) = 213.125 sq ft
Add all walls and ceiling: 124 + 110 + 124 + 110 + 213.125 = 681.125 sq ft
Step 3) Calculate area of windows and door to subtract:
Window 1: width x height = (2)(3) = 6 sq ft
Window 2: width x height = (2)(3) = 6 sq ft
Door: width x height = (8.66667)(3.66667) = 17.77778 sq ft
Add all windows and door: 6 + 6 + 17.77778 = 29.77778 sq ft
Step 4) Calculate total area by subtracting door/windows from walls/ceiling: 681.125 - 29.77778 = 651.3472 sq ft
Step 5) Round to the nearest sq ft = 651 sq ft