Question

I don’t understand it

Answer 1

Diameter is double radius, so it is 36 inches.
Circumference has a formula you must use, and it goes as follows:
C = 2(pi)r
Substitute 18 for the radius and multiply
2(3.1415...)(18)
Round to whatever is asked for. Since I can’t see, I’ll give you a few decimal places.
2(pi)(18) = 113.097335

Answer 2

Radius is half of the Diameter. If the Radius is 18 inches, that means the Diameter must be 36 inches. Circumference is all the way around the circle, That's all I can say about circumference since it was one of the things I never understood fully. I hoped I helped you!