Since circumference=

d,
You would solve for d, using the equation 28.26=

d, since 28.26 is the circumference.
You would end up dividing

on both sides, thus getting the exact answer of 28.26/

as the diameter. If you need an estimated answer, then use a calculator, enter the exact diameter, and round as needed.
(If there's anything wrong/unclear in my explanation then please tell me as this is my first time doing this)
Have a great day!