Answer:
A. The area of a rectangle is equal to all the areas of the small rectangles. The dimensions of the small rectangles are 2a, 3a, and 4a. If you add those areas up, 2a+3a+4a equals the area of the large rectangle.
B. The area of a rectangle equals the width and height. The height of the large rectangle is already shown in the picture, a. The width is seperated into three portions, 2, 3, and 4. To find the total width, add 2+3+4. Now, multiply the width by the height to calculate the area. (2+3+4)a