hi im doing surface area I wondering how do you get the answer . this is the question

Question

Find the total surface area of a rectangular prism having dimension
1) 45cm by 30cm by 20cm

Answer 1

http://www.aaamath.com/geo79_x9.htm

2(45 * 30) + 2(45 * 20) + 2(30 * 20) = ?