Question 42075
Your teacher is right. When we add all surface areas then it is called total surface area. In your question tank is open from top so it has only 5 surface areas not 6 Because top surface area is not available.  So we add like this 
Total surface area= 1(length×width)+2 (length×hight)+2 (width×hight)
Your answer willbe like your techer 1420