1)
You have to find the diameter, which is 2r
You can do it two ways
a) Find the radius first
r =

= √28,26/3,14 = √9 = 3 cm
d = 3*2 = 6 cm
b) Use the formula d =

= √4*26,28/3,14 = √113,04/3,14 = √36 = 6 cm
2)
Use the formula r =

So r = √12,56/3,14 = √4 = 2 cm