The area of a square is calculated suing the formula x^2, where x is a side length of the square. This is just like length times width because a square has equal side lengths.
Take the left side and add the two lengths together because they form one side length of the square. A side length of the square is x + 4.
To find the area using the side length x + 4, you would square the side length. Your answer is (x + 4)^2.