What is the area of a square computer screen with 21 inch sides?

Side of the square computer screen = 21 inches
It is important to minutely read the details that are already given in the question. the most important information that is given in the question is that the computer screen ia square. Based on this information the answer to the question can be easily deduced.
Then
Area of the computer screen = (Side)^2
                                               = (21)^2 inches square
                                               = 441 inches^2
So the area of the square computer screen is 441 square inches. i hope the procedure is clear enough for you to understand.