The Formula for this question is A= L×W because a flat-screen tv is a rectangle and it says that, "These TVs are usually based on the diagonal measurement across the screen" which means its area because it talking about the inside of the shape, not the outside of the shape. So,
A= L×W
A= 26×40
A= 1,040
Your Answer is 1,040!!
PS: If you are not taught this way at school do the way your teacher taught you and I bet you would get the same answer!