Because he cubes the message he wants to send, e=3.
(n,e) is the public key.
C=3, it's the coded message you receive.
M is the message he sent.
Hence, we have
Let d be such that
(you should know that..)
So if you want to get M, calculate modulo n.
Calculating d :
We know that
Either you use a calculator, either you do it by hand. I've tried parts but this yields to numbers I don't like and that are not kind to deal with...
This looks like a draft, but I just tried to make it the simpliest way xD