Diameter of the dartboard as given in the question = 18 inches
Then
Radius of the dartboard will be = 18/2 inches
= 9 inches
We already know that
Circumference of a circle = 2πr
We also know that the value of π = 3.14159
Then
Circumference of the dartboard = 2 * 3.14159 * 9 inches
= 56.548 inches
= 56.55 inches
So the radius of the dartboard is 9 inches and the circumference of the dartboard is 56.55 inches. I hope the procedure is absolutely clear to you.