In part (a), it says that piece B is a parallelogram. To find the area of any parallelogram, we need the base and height. The base is 5 inches as this is the horizontal component of the parallelogram. The height is 4 inches, because 12-8 = 4. Check out the attached image if you're confused how I'm getting the height to be 4.
Once we know the base and height, we multiply them to get 5*4 = 20 square inches, which is the final anwer