3 inches is just 3/12 or 1/4 foot the border minuses area so we have the area=legnth times wdtih the inside length and width are length-2border and width-2border because the border is on both sides
so 6-2 times 1/4=6-0.5=5.5 4-2 times 1/4=4-0.5=3.5 so the demotions inside the border are 5.5ft and 3.5ft and the area is 19.25 square feet
