radius
is half of the circle
diameter
is the whole thing inside the circle
since it is a radius, and it is a half of the full circle, we need to double out that number, and multiply by 2.
so here, you said it is 28
take 28, and multiply by 2, if you do every thing correctly, you should get:
56
so then answer would be 56 centimeters.
Hope that this helps you! =)