Since it is about only the outer part of the aquarium which is made of glasses we need to calculate the total surface area.
Surface area of the base = 4*3 = 12 square feet
Surface area of the shorter length of wall = 3*2 = 6 square feet since it has another wall in opposite side. It has to be multiplied with 2. 6*2 = 12 square feet
Surface area of the longer length of wall = 4*2 = 8 square feet since it has another wall in opposite side. It has to be multiplied with 2. 8*2 = 16 square feet
Therefore the total surface area that need to be covered with glasses 12+12+16 = 40 square feet
