+770 I'm assuming by c you mean the circumference, and d is the diameter.
The circumference of any circle is the diameter times pi.
Here, I'll write pi as 3.14 because if I was to write it out it would take forever (quite literally).
C = D * 3.14
7.85 m = D * 3.14
D = 7.85 / 3.14 = 2.5 m
The diameter was 2.5 m :D
