d = 14 mm the diameter of the coin

Assuming the coin is a circle we the area of each of the coin's sides equals the area of a circle with diameter d = 14 mm:

A = (d/2)^2pi

A = (14/2)^2pi

pi = 3.14

A = 153.86 mm^2

The exact area of each side of the coin is 153.86 mm^2.