You are subtracting 3 each time. (-3)
1.15¹⁰=4.0456, the growth rate, so the increase is 3.0456 or 305%.
Keeping in mind that the area of a rectangle is simply width * length, if we get the area of the larger rectangle, and then
subtract the area of the smaller rectangle, we're in effect making a hole in the larger rectangle's area and thus what's leftover is the shaded area.