Arectangle is placed around a semicircle as shown below. the width of the rectangle is 4mm . find the area of the shaded region.

Formulas:
Area_semicircle = Πr2/2
Area_rectangle = lw

Given:
Semicircle is engraved in the rectangle
length_rectangle = 8

Assumption:
Diameter of semicircle is equivalent to length of rectangle
diameter_semicircle = length_rectangle
diameter_semicircle = 8

Based on assumption, radius_semicircle is equal to the width of the rectangle.
diameter_semicircle/2 = radius_semicircle
radius_semicircle = 4 = width of rectangle

Solve:
Area_rectangle = l*w = 4*8 = 32
Area_semicircle = Πr2/2 = Π(4)2/2 = 8Π = 25.12

Area_rectangle - Area_semicircle = 32 - 25.12 = 6.88

0.4 / 100 * 69000 = 276 acres of old-growth trees.