use the formula 2πr
- 106.76 
 - 300.96
 - 2×3.14×(1/2)×39 = 122.46
 - 78.5
 
the c bit I did the whole process cuz the diameter is given and for the d bit i thought what you were trying to say is 25 diameter.
hope it helps and don't forget to put the units wherever necessary.!!!!