The formula for the circumference of a circle is C = 2πr, where r is the radius and π (pi) is approximately equal to 3.14.
In this case, the radius is given as 4.2 millimeters, so we can plug this into the formula to get:
C = 2πr
C = 2 × 3.14 × 4.2
C ≈ 26.39
Rounding to the nearest hundredth, the button's circumference is approximately 26.39 millimeters.