Question 551999
Circumference = 2πr
r = radius 
1/2 diameter = r
so half 90 mm => 45 mm
Plug into 2πr => 2π(45)
Simplify by multiplying 45*2 => 90π

Thus your Circumference is 90π mm or ~282.74 mm

P.S. You noticed I first divided the diameter by two then later multiplied it by 2. This is unnecessary however it is how its written in most textbooks. For your future reference the Circumference of a circle is just diameter*pi