The large square at the left of the picture is divided into four pieces. At the lower left is a square labelled A whose sides are each 7 inches.
At the upper right is a square labelled D whose sides are each 3 inches.

The area of square A is square inches.

The area of square D is square inches.

Rectangle C is 7 inches long and 3 inches high so its area is square inches.

Rectangle B is 3 inches long and 7 inches high so its area is square inches.

The area of the large square at the left of the picture is square inches.

To form the rectangle at the right of the picture, first remove square D which leaves a figure shaped somwhat like an L.
Now notice that if we turn C on its side it will look just like B so we can place it next to B.

The length of the rectangle at the right of the picture is inches.

The area of the rectangle at the right of the picture is square inches.

Now, we have removed a square of area $3^2$ square inches from a square of area $10^2$ square inches and
the area that was left formed a rectangle of area $(10+3)\times (10-3)$ square inches.

Suppose the square containing A, B, C, and D at the left of the picture had sides of length 100 inches and the square D had
sides of length 3 inches. Then the area of the rectangle at the right of the picture would have been square inches.