Answer:
75.4mm
Explanation:
Circumference is the distance around a circle (just like perimeter, but a special word just for circles)
You find circumference by this formula:
C = pi•d
where d is the diameter.
They gave you the diameter in your problem, 24mm
C = pi•24
C = 75.39822
Rounded to the nearest tenth is 75.4mm
(also 24pi is a perfectly good, exact answer or 75.40mm if you need it rounded to two decimal places)