To solve this problem we can find the area of the whole rectangle, then find the area of the smaller white rectangle inside the whole rectangle. Lastly, we can subtract the area of the smaller rectangle from the area of the whole rectangle which leaves us with the area of the shaded region.
Area of whole rectangle = length x width
= (3x + 6) (2x + 4)
= 6x squared + 24x + 24
Area of small rectangle = length x width
= (x-3) (x - 1)
= x squared - 4x + 3
Area of shaded region = area of whole rectangle - area of small rectangle
= 6x squared + 24x + 24 - x squared + 4x - 3
= 5x squared + 28x + 21