Since a rectangular box or tank has opposite sides which are equal we calculate each unique sides area then add them up together and finally multiply by two to find the total surface area. Check out this colorful and fun video and learn how to find the area of a Rectangle. To find the area of the rectangle just multiply the two edges together.
A side length.
Find the area of each face that you see and then multiply each area by two to account for the face on the opposite side. Add those two parts together and you have the formula for the surface area of a cylinder. The area of a rectangle is the product of the two sides. S tot total surface area.